ANATOMY OF AN IMPROVISER: THE STYLE OF NAILOR AZEVEDO “PROVETA
Manuel Silveira Falleiros
2.1. Creative Process
As presented in the previous chapter, Nailor’s musical trajectory—revealing a rich musical experience—also demonstrates how his experiences coalesced to form his concepts about music and, consequently, his musical poetics.
A musical formation rich in variety, almost picturesque yet simultaneously so common among Brazilian wind instrumentalists, provides us with the opportunity to investigate how, within this context, musical exponents such as Nailor can emerge.
As we have seen, from the outset of his professional career, Nailor distinguished himself not only as a performer on his instrument but also as an arranger, composer, and improviser.
“[…] in addition to mastering the intricate nuances of harmony and orchestration, he also possesses immense natural talent for finding well-constructed phrases, precise rhythm […]”⁵⁸
This natural talent, however, is the result of what Nailor acquired over years through formal and informal studies across diverse areas of musical knowledge: instrumental technique, interpretation, orchestration, composition, improvisation, music history, and aural perception—without confining himself to the boundaries of any single musical genre or style.
His musical studies include a series of transcriptions of improvised solos by prominent jazz musicians (a common practice in jazz pedagogy methodology), but also, in order to deepen his understanding of other styles, Nailor transcribed symphonic works. In fact, he transcribed and analyzed everything that stimulated his curiosity:
“Phil Woods, certainly; Dexter Gordon; Johnny Griffin; Charlie Parker—almost all his solos; John Coltrane; Eddie Daniels on clarinet—he transcribed all of that. And also, for example, excerpts from a symphony that interested him: he would put on the record and start figuring it out. He would transcribe an introduction, something we always did; we used to seek out many classical music scores to listen to and follow along. And Proveta not only did this but also analyzed: he would sit down and analyze; he wanted to analyze everything.”⁵⁹
In the previous chapter, we discussed the importance of transcription as an activity for acquiring musical knowledge through the formation of a repertoire and a field of samples. These practices helped Nailor infer the functioning of musical structures. Nor can we overlook whether the variety of styles he transcribed may have contributed to the richness of his musical creation.
In the same manner that he uses transcription of works and musical excerpts to cast an analytical gaze upon musical aspects, Nailor employs improvisation as a tool that enables the integrated application of this acquired knowledge through instrumental skill. This procedure plays a central role in refining musical structures, absorbing concepts and stylistic idiosyncrasies, and developing technical fluency on the instrument:
“If you want to understand the harmony of choro, improvise some baixaria⁶⁰ lines on your instrument.”⁶¹
Nailor refers to baixaria—a line that outlines harmony in an ornamented manner, highly characteristic of the choro style (typically performed on the seven-string guitar). This exercise functions in an interesting way because it simultaneously serves multiple aspects of musical development: while one learns the harmonic cadence of a given piece, one also makes considerations about style that contribute to interpretation, and it further demands instrumental technique that fosters rapid reasoning and conditioned reflex.
Improvisation is a musical activity commonly characterized as a spontaneous practice. However, many composers across diverse genres employ improvisation as a means to scour their minds in search of ideas, themes, developments—in short, possibilities. They thus use improvisation as a tool for other musical poetics.
Nearly the entire tradition of Western music finds in improvisational practice a source of musical creation. J. S. Bach composed a significant portion of his works from improvisations on the organ and harpsichord (such as the Goldberg Variations, for example). Mozart, like numerous composers of the Classical period, also practiced improvisation, even employing it as an integral part of musical form—for instance, in the cadenzas of piano concertos. The musician improvised within the parameters of harmony, counterpoint, and musical form. In this way, it was possible to obtain a result that was spontaneous yet also coherent and organized.⁶²
At this point, we may distinguish two forms in which improvisation presents itself. First: it may serve as a tool for musical creation, much like a sketchbook filled with unfinished croquis that may later be transferred to a large canvas and endowed with appropriate colors and background. Second: improvisation may be understood as the artistic act itself, immersed in the present. It is more likely that the character of spontaneity associated with improvisation relates to the second definition presented above.
The sense of spontaneity present in musical improvisation should not be read literally as a musical practice that occurs purely left to chance—at least not with reference to the object of our research. In fact, it is far from that. Although indeterminacy, at varying levels, is what unites diverse concepts of improvisation across distinct eras and cultures, in this case it does not refer to arbitrary or total indeterminacy, but rather to the indeterminacy of specific parameters to the detriment of others that remain stable. Moreover, it does not entail a combination of random, aleatory sounds, but a deliberate selection of note combinations in pursuit of musical coherence. The quality of Nailor’s improvisation depends, in the first instance, on his musical skills—developed and refined through practice—and subsequently on his experience. Therefore, although improvisation is sometimes considered a spontaneous practice, it may play the role of organizing, according to the rules of musical discourse itself, the material of memory toward a unified direction.
John Kratus, in his study on the teaching of improvisation, affirms that “[…] all improvisations result from a deliberate intention, not from chance, to create musical sounds.”⁶³ For all these reasons, we conclude that Nailor is conscious of the final outcome of his improvisations because he clearly knows what he will play while improvising.
Thus, we may consider that the distinct categories of Nailor’s musical practice—improvisation, composition, arranging, interpretation—are all manifestations of a single creative desire oriented toward his will to express himself artistically. These categories obviously retain their distinctions, although they all stem from the same creative impulse. For Nailor, the difference between composition and improvisation lies in the fact that the former is prone to subsequent revision, whereas the latter is prone to prior preparation.
If improvisation is prone to a certain preparatory groundwork, we conclude that Nailor’s natural talent is, in fact, also the result of extensive work in developing his musical skills. Therefore, we must examine whether it is possible to determine at what level of development Nailor’s improvisation stands, in order to ascertain the degree of refinement he has achieved in this skill. Improvisation plays a prominent role, considering that it participates in his musical development not only as expression per se but also as a tool for acquiring knowledge and generating musical ideas for other poetics.
In order to demonstrate that his improvisational skills operate at a highly developed level, we will employ the model of musical development proposed by John Kratus in his text A Developmental Approach to Teaching Music Improvisation⁶⁴. This text addresses the question of musical development observed in the teaching of improvisation. His study is based on earlier authors who generally share the idea that musical development presents a sequence of cumulative stages, which can be observed and determined.
In this model, Kratus proposes an evolution in the musical activity of improvisation, divided into seven stages possessing distinct characteristics. Namely: Exploration, Process-Oriented, Product-Oriented, Fluid, Structural, Stylistic, and Personal.
The first phase, called Exploration, is a pre-improvisational stage in which the student cannot predict the sound they will produce; improvisation is devoid of purpose and structural constraints. In the second stage, Process-Oriented, the student is capable of imagining and determining sounds; however, there is still no concern for producing something comprehensible to those around them. This concern emerges in the following stage, Product-Oriented, in which the student begins to seek some structuring of their improvisation, thus enabling their music to be shared with others. A further technical step on the instrument is taken by the student in the next phase, called Fluid. With more automated technique, the student gains fluency in scales, rhythms, etc., from the point at which they feel the need for these elements to transform their ideas into sound. In the subsequent stage, called Structural, the student employs diverse strategies aimed at creating form in improvisation, such as developing ideas, generating interest through tension and release, and transitioning between musical ideas. The penultimate stage, Stylistic improvisation, is considered by the author to be the level most musicians attain. At this stage, the musician must possess detailed knowledge of a particular musical language and the ability to apply it fluently in their improvisations—that is, the manipulation of resources that characterize a style is, at this stage, complete. It is also at this stage that one learns a vast repertoire of pieces from one or multiple styles, which serves to provide a foundation for improvisation. At this stage, the improviser is already capable of applying clichés and emulating the style of other musicians. Stimulated to continue exploring these resources, the improviser needs to extrapolate certain rules to further expand these concepts. It is in the final stage, called Personal, that the improviser possesses all the skills that lead to the creation of a unique identity, derived from an original expression. According to the author, the emergence of a new, original style of improvising is generally observed at the confluence of distinct styles.
Based on what we observed in the previous chapter, Nailor underwent certain phases of study that align with those proposed by Kratus. Succinctly, we present the comparison:
Born into a musical family, with the presence of diverse musical instruments in his home, it is clear that the Exploration phase in improvisation certainly occurred, as did the Process-Oriented phase. However, due to the lack of precise accounts, we cannot yet clearly determine, from the interviews, any moment associated with these first two phases. Only a few brief passages provide clues to the occurrence of these stages: in his father’s early lessons, Nailor freely explored musical notes on the accordion, seeking sounds but not meanings. Subsequently, when he was asked to accumulate repertoire and create musical sounds related to an original theme, this shows that Nailor knew what sound should emerge from the instrument; this level of more conscious manipulation aligns with the phase described as Process-Oriented.
It is clear, however, that the Product-Oriented phase presents itself at the moment when Nailor is required by his father to transcribe solos and present them. As we also saw in other interviews, Nailor, in his childhood, was recognized as a musician by members of his community and was always in possession of his instrument, playing for people. This indicates that there is already a concern with creating something that aligns with shared musical values in society, which in turn reinforces the connection to the terms of this developmental stage.
The phase described as Fluid improvisation can be observed when repertoire learning becomes extensive for Nailor, upon his effective entry into dance bands in the interior of São Paulo state, at which point he acquires greater familiarity with scales and rhythms. This also occurs through the automatization of these resources, which we observe when Nailor assumes the position of soloist in Sílvio Mazzuca’s orchestra.
Structural. As we have seen, this phase is marked by concern with the macro-structure of the music one decides to create. We observe that Nailor, in Sílvio Mazzuca’s Orchestra, received instructions from the Maestro himself to begin seeking elements of interest in his solos. He was also assisted by colleagues who provided him with books of phrases, scales, and patterns, as well as diverse exercises, which ultimately gave form to and expanded his repertoire of musical ideas.
We verify the Stylistic phase in Nailor’s concern with knowing the stylistic traits of a given style. With his participation in the band of the 150 Night Club, and the formation of the bands Aquarius and Sambop, we see that these were spaces in which the ability to interpret distinct styles in popular music was required.
Therefore, based on a comparison between the aspects that define each of the stages of musical development, we observe that the sum of Nailor’s skills and experiences places him currently in the final stage of development (Personal). Another argument suggesting that he fits this phase is that Nailor fuses elements from distinct musical styles in favor of his own creation.
The fusion of styles occurs through the displacement of musical elements into spaces where, more traditionally, they would not be found. Although conserving the same concept of improvisation, Nailor certainly does not improvise today in the same manner as when he was eight years old. This is due to two reasons: the first refers to his musical evolution, described earlier using John Kratus’s model of musical development. The second arises from the need to expand this concept of improvisation so as to incorporate elements from other musical cultures. In other words, his musical development necessitated a modification in his concept of improvisation.
Having contact with diverse styles and ways of improvising, the concept of improvisation as variation becomes limited for Nailor. How can one assert that John Coltrane’s improvisation on Giant Steps is based on variations of the theme (even if one says they are extensively developed or complex), when precisely the idea is that no element should appear to be repeated?
For this reason, Nailor chooses to operate in an environment that is safer for his own concepts:
“The problem is not disliking jazz; it is speaking the wrong thing at the wrong time—not having knowledge of this. That is why I prefer to play a choro…”⁶⁵
The way to avoid undermining this concept of improvisation related to variation, while simultaneously allowing room for the inclusion of other musical materials, was to classify other forms of improvising as distinct categories of improvisation in form, without allowing them to absorb the musical content. This is because, although he moves across other musical genres, Nailor is expressly concerned with being as legitimate as possible in his art. Thus, Nailor seeks to adapt musical knowledge to his manner of playing based on choro. Therefore, we will see that he manages to utilize…
The testimony of Mozar Terra, at the moment when we questioned him about the appearance of elements not very usual in the more classical context of the choro genre, reinforces that:
“Proveta knows how to use the dom-dim scale⁶⁶, but as a diminished [scale]; it belongs to the Brazilian language, it comes from choro.”⁶⁷
This represents a strategy that demonstrates the astuteness of including a type of material that may sometimes serve to showcase the quality of one’s skills, without, however, losing dialogue with what is most particular to him—thus serving, in this way, to legitimize his production.
It is common in the choro genre for phrasing to be executed with ascending or descending diminished arpeggios, with the latter appearing even more commonplace. We find the diminished scale used with a melodic contour that can be theoretically interpreted as chromatic approaches (embellishments) over a diminished chord. Knowing that the dominant-diminished scale shares the same intervals as the diminished scale, the designation of one or the other depends only on the harmonic material occurring and the rhythmic position of its notes. With this, we have these three elements unfolding within the same space in a well-articulated manner.
This demonstrates that, although at times Nailor’s initial melodic design is based primarily on the chorostyle, he remains open to inclusions of materials derived from other genres.
We have thus seen an example of the application of new musical material without loss of genre characteristics. This type of procedure may justify the assertion that Nailor operates at a high level of development. However, we still need to learn more about his concept of improvisation.
In the previous chapter, we saw that Nailor acquired knowledge about improvisation, as well as about other areas of music, stemming from often informal learning, in which his father’s role as teacher was prominent. We verified in the first chapter that Nailor’s concept of improvisation is based on the idea of melodic variation, and therefore we will focus on understanding this idea. Schoenberg offers the following definition of variation:
“Variation means change: but changing every element produces something strange, incoherent, and illogical […] variation will require the alteration of some less important factors and the conservation of others that are more important […].”⁶⁸
This definition is useful for ordering and directing the elements that produce variation (by suggesting the categorization of these elements according to their greater or lesser importance). Such a definition will initially guide us in seeking to understand the meaning of variation, but we will need further resources to comprehend what this signifies in Nailor’s own terms.
Another interpretation, regarding improvisation as interpretive variation on a theme, presents the concept of melodic variation closer to that taught to Nailor by his father:
“Variation after repetition is indispensable nowadays. This is expected of every performer. The audience demands that practically every idea be repeated in an altered manner […].”⁶⁹
“You play a melody; when you return [repeat the theme], you play another melody, but stay close to the first one.” That was his lesson. […] He knew that improvisation was a melody. So he would say: “Stay close to the first one; don’t go far, because you won’t be able to return.”⁷⁰
In the first definition above, we see the concept of variation as the application of changes in the manner of re-presenting the idea (theme) through alterations in its form; however, a certain basic structure must be maintained, thus guaranteeing connection with the original. From this perspective, the three definitions presented share the same idea.
But how to justify improvisations in which elements of the original theme can hardly be identified?
According to Nailor, his variation does not occur only in the literal field of notes; rather, in his view, the way to go further without losing the original reference is to create based on the sensations and adjectives⁷¹ that the original theme provides him.
What we can actually identify are the ways in which this relationship between the original and the variation manifests in music. Since musical genres are normally linked to certain behaviors, it is necessary to know which style is most determinant for Nailor’s musical behaviors.
As we have seen, the foundation of his musical thought comes from choro. It is upon the framework of chorothat Nailor shapes elements derived from other musical genres. Nailor actively participates in this language and employs it with great authority, even when expanding it to levels that surpass the formalities of the style. Therefore, we may say that he has absorbed typical stylistic behaviors through his entire musical experience. If this is fundamental to his artistic conception, then we need to understand the peculiarities of the style.
Its etymology is highly varied and, in fact, reveals nothing profoundly insightful about the style itself. Some historians believe the word derives from xolo, a dance of enslaved people; for others, it refers to the languid or tearful manner in which these musicians played; or perhaps there is a connection to the charamela, a double-reed instrument with a strident sound in which Black musicians notably specialized, and because of this were known as choromelleyros; furthermore, choro became known as a synonym for seresta, for open-air dances, with expressions such as cair no choro (“to fall into the choro“), meaning to dance said dance. It is also probable that the name arises from the confluence of all these examples together.
Most historians agree that choro originated around 1870. However, at that time, since it was an incipient, not yet consolidated style, compositions did not present themselves in the manner we know today. In reality, the term choro then referred to a particular manner in which Brazilian musicians interpreted the European repertoire of the time—a repertoire consisting of pieces in styles such as waltz, schottische, mazurka, tango, habanera, and especially polka. The Brazilianized manner of performing this repertoire was nothing other than the application, initially to the accompaniment, of characteristic accentuations derived from those musical manifestations brought by enslaved Black people that took on distinct form in Brazil, such as lundu or samba. Due to this fact, it is probable that the melody, by necessity of sharing and dialoguing with the characteristic accentuation of the accompaniment, also underwent the application of these accentuations in its rhythm. With greater emphasis on these accentuations, the rhythm gradually shifts as well; rhythmic figures are attracted by the accentuations. What caused the style’s development to be exponentialized was the fact that this repertoire was often transmitted orally—that is, a melody that received a personal interpretation of Brazilian-style accentuations would become established as the rule for whoever learned it subsequently, and what was once a variation became the crystallized form.
“Oral tradition is an important component within numerous musical genres worldwide. Their complex musical structures—including highly developed systems of tuning and rhythm, and brilliant performance techniques that define them—are frequently learned aurally (wholly or partially) and subsequently improvised. […] Their repertoire is partially preserved in notation, but the essence of the music is locked in oral tradition and intimately linked to the performer’s creative ability.”⁷²
We must also clarify that the term improvisation has distinct meanings in different eras, so that we do not fall into the misfortune of launching unfounded theories in a desperate attempt to approximate what should not be approximated. At the time when choro was disseminated as a manner of playing, the meaning of improvisation differed from today’s, mainly due to the nearly immediate parallel currently established between this term and jazz improvisation. In fact, improvisation referred to the very manner in which music generally occurred—the manner in which music was made without notation, without ceremony, altered according to the performer’s taste (with the intent of demonstrating their expression), without rehearsal, without elaborate arrangement, without prior programming—that is, made there, in the moment, with what one had at hand.
Improvisation, therefore, in the context of choro, has its unique character. This particular manner of improvisation shares some of the more general concepts with other forms of improvisation in music. Up to this point, we have been able, for purposes of contextualization, to compare choro improvisation to other styles. However, improvisation in choro follows its own development, with its own rules, and this, at this moment, removes our right to insist on these parallels. Improvisation in choro is marked by the characteristics of the style and by its own performance practices.
Therefore, it is important to emphasize that, by the very practice of this musical style, the result is a melodic line that relates to the harmonic fabric in a manner very distinct from the comparative examples we have presented thus far. In the practice of choro, harmony must, in a certain way, permeate the melody, and examples illustrating this fact are not rare. We see this in the practice of the style, in which those responsible for harmonizing—without formal knowledge of harmony, nor able to rely on notation—were constantly tested in their ability to conduct harmony based solely on experience and guided by intuition. And in this, also as typical behavior of the style, soloists applied unexpected modulations, among other capricious devices. Examples even appear in composition titles, such as Apanhei-te Cavaquinho by Ernesto Nazareth, which alludes to the possible error committed by the cavaquinho player in his attempt to conduct harmony without notation, guided by intuition.
Choro is heir to a more accelerated urban development, from around 1790 onward, primarily in Rio de Janeiro, which brought cultural diversity through the unrestrained expansion of the population. Popular classes, lacking the piano to animate their festivities—and even more distinguished families—found in the choro ensemble, called a regional, an adequate alternative suited to their investment possibilities. Thus, the same group that sometimes played in a more restrained manner inside a family home could exaggerate syncopations when playing on the street or in a bar. The interesting aspect of this fact is that this group did not necessarily need to have an exactly fixed formation, always with the same instruments, but only that the instruments assumed melodic or accompaniment functions. It was not uncommon for a trombonist, or any other instrumentalist, to join the other musicians and play what they could: if they knew the theme of the piece, they stated it; but otherwise, they unfolded improvised countermelodies. Chorogroups, or rodas de choro, have always been open to the participation of other instruments; the function of the instrument within the group was based not only on its most traditional role—in fact, the instrumentalist adapted to the group by playing according to what their skills allowed.
This seemingly disordered gathering of musicians creates the need for common codes and a known repertoire, and furthermore demands from the instrumentalist a flexibility to adapt to the situation: both regarding interpretation appropriate to the venue and regarding their function in relation to the instrumentation encountered. To this, too, we may refer as improvisation, in the context of choro.
Improvisation in choro has never been dissociated from the performer’s desire to demonstrate their instrumental skills through virtuosic display, in a kind of internal competition. This practice is a form of demonstrating power in a game of prestige among the musicians themselves. And this is what, in choro, ends up determining the type of ornamentation—generally virtuosic, dazzling, and bold—that is applied to these melodic lines. The two cases—in the improvised execution of accompaniment and in the ornaments of the melodic line—are representative of the use of astuteness in demonstrating technical-musical superiority, on the part of each.
The melodic bass line, later called baixaria, relies primarily on harmony for its creation but sometimes transgresses it. This line that the bass describes—whether executed on the seven-string guitar, the ophicleide, the tenor saxophone, or even the trombone—has always played a prominent role in characterizing the style. In the procedure of its execution, it is very common for inversions in chords to occur in order to make them more linear—that is, lines that are conducted more by stepwise motion than by the quartal leaps inherent to harmonic cadence. This line, which interacted with the melody, was generally not written, and its quality depended on the competence of the instrumentalist who executed it. This practice is one of the characteristic elements of the style.
Choro as a musical genre, in its form, is constituted of three distinct parts, each in a different key, which generally articulate through relative or neighboring tones. It was common for the third part of the piece to be created by the musicians—improvised—and if this did not occur, it fell to the representatives of harmony (guitars, for example) to maintain only the harmonic progression. From this arose the practice of improvising the third part (and not of improvising within the third part, in this context).
The improvised melody may develop to a high degree of complexity, but even so, it generally retains some element by which the initial material can be identified, such as the repetition of a refrain, a characteristic rhythmic figure, or a melodic contour.
In some cases, the melodic interpretation itself in the context of choro—when more daring or elaborate, in its alterations of tempo, notes, and ornamentation—is called improvisation.
In short, we conclude that all these practices compose the meaning of improvisation in choro. To improvise means, therefore, to adapt tools for other uses by force of circumstances. Since choro is the genre of Brazilian music that stands out for its instrumental force and improvisation, we can verify the more or less direct influence of its concepts on the Brazilian instrumentalist.
We can observe how Nailor’s musical thought employs the concepts of choro presented above through the characteristics of his creative process. One of these characteristics of his musical activity is that, throughout the entire creative process—whether in his solos, arrangements, or compositions—Nailor does not use harmonic instruments but imagines a melodic path that outlines the harmony:
“Proveta does not use harmony to play […] it’s not that he doesn’t use it; he sees harmony as if he already saw a melody on top of it, and was constructing things on top of the melody.”⁷³
“When I write, I try to guess the line that each [musician] would like to play […]”
This does not surprise us, considering that his musical learning was based almost exclusively on melodic activity, and that his instruments of study were the saxophone and clarinet. His practice included chord sequences, complex structures, inversions, extensions, and every type of harmonic material of which he could become aware. Therefore, the understanding of harmonic elements always passed through the lens of his saxophone—a melodic instrument—and thus, from the perspective of his experiences, taking harmony as the primary vehicle for his musical creation does not fully reflect his manner of thinking. In other words, for Nailor, the harmonic path reveals itself and makes sense when presented in melodic or contrapuntal⁷⁴ form.
For Walmir Gil, Proveta’s direct influence is choro. That is to say, if musical thought was developed through melody and his greatest influence is choro, then it is probable that the melodic line of choro is a representative element in his musical conception. However, we know that not only choro but other musical genres were present in his formation. In other words, we find reinforcement for our hypothesis that the foundation for the construction of improvisation for Nailor is melodic motion.
Earlier, we demonstrated that Nailor’s life history and musical experience link him profoundly to choro—whether through family tradition, practical knowledge derived from repertoire, or his constant engagement with this style. As we were able to ascertain above, there are no specific studies for improvisation in this genre; what exists is the study of music: repertoire, technical skills on the instrument, knowledge of harmony, arranging, perception, etc. It is such knowledge that ultimately contributes to the quality of improvisation, for the more depth in theory and practice the instrumentalist can obtain, the more resources they will have at the moment they need to adapt to the musical environment to which they are exposed—that is, to improvise.
For Nailor, the act of improvisation is one of the consequences of his need to make music. It is impossible to dissociate his musical experience from his existence; Nailor even affirms that music is something that is part of his organism, so great is his involvement with it. This involvement is best represented, as we have seen, among his musical activities, by improvisation.
Nailor affirms that his ideal of improvisation is one that presents musical content in which there is a balance between the use of advanced musical knowledge and the expression that represents his roots, with the intent of giving depth to his work.
We finally gather, in a focused manner, as a conclusion to this chapter, the most relevant aspects of Nailor’s creative process: a) Nailor operates at the highest degree of proficiency in improvisation, according to the comparison made with specialized studies; b) His concept of improvisation is based on the idea of variation; c) Nailor employs in improvisation techniques derived from diverse areas of his musical knowledge.
Finally, to better illustrate Nailor’s creative process, we will use as examples analyses of transcriptions of his improvised solos.