The language

A project is ONE document. There are no blocks and no play keyword.

• An unindented expression line is a playable segment.
• Indented lines continue the segment above them.
• A line containing ::= is a rule definition.
• A line whose first word is a command word is a command.
• % starts a comment, to end of line. %% or more is a zero-duration grid symbol, not a comment.

Blank lines are spacing, any number of them.

bpm 60
key c4 maj
 
Fig ::= 1 2 3
 
Fig
Fig Fig

The only automatic composite is the whole document's: a file that states no expression sounds the LAST rule it defines. Definitions elsewhere sound nothing of their own.

bpm 60
key c4 maj
 
A ::= 1 2
B ::= 3 4
% no expression line: the document sounds B

One per line.

bpm counts QUARTER notes and one unit is a SIXTEENTH, so a unit is 15/bpm seconds and four units make a beat.

bpm 60
key c4 maj
meter 4
unit 1/4
 
assert 1 == c4
assert 1 _4 == c4 _4

Notes are lowercase scientific pitch: c4 is middle C. Accidentals suffix the letter, and double accidentals are allowed: f#3, eb5, c##2, gbb3.

Degrees are bare numbers, resolved against the current key and mode. They auto-wrap over the whole integer line, so 8 is the octave above 1 and 0 is the seventh below it.

• Octave prefixes curry: '3 is up one, ''3 up two, .5 down one.
• Accidental suffixes inflect chromatically: 6#, 7b.
• m60 is absolute MIDI in any mode.

bpm 60
key c4 maj
 
assert 1 2 3 == c4 d4 e4
assert 8 == '1
assert '1 == c5
assert .7 == b3
assert 4# == f#4
assert m60 == c4

Rests are -. A run of dashes is still ONE rest; -*4 is the canonical spelling of four units of silence.

Relative pitch counts from the last written pitch: i<n> in semitones, s<n> in scale steps. It is threaded left to right.

bpm 60
key c4 maj
 
assert c4 i4 i3 == c4 e4 g4
assert c4 s1 s1 == c4 d4 e4

Anchors set what relatives count from WITHOUT sounding: !c4 has zero duration. No mark may follow one.

bpm 60
key c4 maj
 
assert !c4 i4 == e4

Chords are one token, <root><Quality>[$voicing], sounding together in one slot. Roots may be absolute (cMaj) or degrees (1Maj, 5Dom7), and octave prefixes and accidentals compose (.1Maj, 4#Min).

Qualities compose: a base (Maj Min Dim Aug Dom Sus2 Sus4), an extension (6 7 9 11 13), and alterations (b5 #5 b9 #9 #11 b13).

Dia builds the stack FROM the scale, so the key decides the quality.

bpm 60
key c4 maj
 
assert 1Maj == {c4, e4, g4}
assert 1Dia == 1Maj
assert 2Dia == 2Min
assert 7Dia == 7Dim

A $ voicing lists members bottom to top; a member the chord lacks is an error, never a silent no-op.

The duration of an item is

dur = (1 + Σ '+') × ∏('_') ÷ ∏('/')

bpm 60
key c4 maj
 
assert 1 _2 == 1 +1
assert 1 _2 /2 == 1
assert 1*2 == 1 1

* groups its copies, so a mark after it covers the whole group: kick*4 /2 is four kicks inside two units.

A mark after an item transforms it. After a rule reference the transform covers the WHOLE expansion.

bpm 60
key c4 maj
 
Fig ::= 1 2 3
 
assert Fig~ == 3 2 1
assert Fig@2 == d4 e4 f#4
assert Fig@~1 == 2 3 4

They are component-wise and order-insensitive among themselves, but the engine reflects BEFORE transposing, so a ^ at a use site negates a fold's @k. Chromatic folds do not commute with @~ marks (stepShift(x+k,n) ≠ stepShift(x,n)+k), which is why a fold whose uses carry @~ is refused.

Nested ^ cancel. Drums and rests are unchanged by ^ and @.

NAME ::= body. Uppercase names are rules; everything else is a terminal.

• = separates alternatives, and extra alternatives may go on their own lines starting with =.
• EBNF postfix: & zero-or-more, ? a timed optional slot, ( ) groups. One-or-more is x x&. * and + are duration marks, not repetition.
• Name#n pins a reference to the n-th derivation instead of ranging over all of them.

bpm 60
key c4 maj
 
Bar  ::= 1 2 3 4
Bar*2

A rule the document never states sounds nothing.

A […] suffix slices the result python-style, over ITEMS not tokens.

bpm 60
key c4 maj
 
Row ::= 1 2 3 4 5
 
assert Row[1:3] == 2 3
assert Row[::-1] == 5 4 3 2 1
assert Row[-1] == 5
assert Row[:2] == 1 2

A reference may override one item with \<index> <replacement>. The index is ZERO-BASED — \0 is the first item — and it counts what the reference PLAYS, not its body's tokens, so it survives a later pass factoring that body into references of its own.

bpm 60
key c4 maj
 
Row ::= 1 2 3
 
assert Row \0 5 == 5 2 3
assert Row \2 5 == 1 2 5

The replacement is ONE element. Brace it if it carries a mark of its own, or the mark is read as the reference's:

bpm 60
key c4 maj
 
Row ::= 1 2 3
 
assert Row \0 {5 _2} == 5 _2 2 3

Slices and overrides differ in what counts as an item: in a SLICE a marked nested reference counts as one item.

An override must come BEFORE any mark, and putting it after fails silently. Row@2 \1 {7 _4} does not transpose and does not override — it gives back the bare rule, with no error. Both marks are discarded. The form that works puts the override inside what the mark covers:

bpm 60
key c4 maj
 
Row ::= 1 2 3
 
% the override reaches the replacement, and the mark reaches both
assert {Row \1 {7 _4}}@2 == d4 {c#5 _4} f#4

The two contexts fail differently, which is worth knowing. As a STATEMENT, Row@2 \1 {7 _4} sounds the bare rule and says nothing. Inside an assert the same text is rejected — unknown symbol(s): \1 — so a claim pinned by an assert catches what a statement swallows. That is the general case for preferring an assert over a sentence.

{A, B} sounds its fields together. Fields are separated by ,.

bpm 60
key c4 maj
 
assert {1, 3, 5} == 1Maj

Direction toggles and relative pitch are inherited by {} fields at entry; a chord passes on its first field's pitch.

| … | stretches what is between two bars to exactly ONE measure of the current time signature. | a b | c d e | is two equal measures with different subdivisions.

Bars scope per source line, and a : directly after | ends the run.

A barred body cannot be referred to in part. A slice drops the bars, so R[1:3] of a barred R sounds its items at consecutive positions, measure-aligned slices included.

bpm 60
meter 4
unit 1/4
key c4 maj
 
Two ::= | 1 2 | 3 4 5 |
Two

A rule may take arguments. $1 is a hole the argument fills; a bare reference spreads and tiles over the argument.

bpm 60
key c4 maj
 
Swing ::= $1 _3 /2 $1 /2
Line  ::= 1 2 3 4
 
Swing(Line)

Slicing a call-bodied rule returns everything — a known sharp edge.

Exactly one mark's worth of difference, and nothing else:

• the same figure TRANSPOSED — @k, @~k
• the same figure at a constant DURATION RATIO — _n, /n, composed as _3 /2
• each PRODUCTION a generative rule states

Two spellings that play the same (time, midi, dur) events are the same music; assert is how the document says so, and the engine checks it.