LilyPond Notation Reference: 5.6.1 Substitution function syntax

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Making a function that substitutes a variable into LilyPond code is easy. The general form of these functions is

function =
#(define-music-function
     (arg1 arg2 …)
     (type1? type2? …)
   #{
     …music…
   #})

where

`argN`	`n`th argument
`typeN?`	a scheme type predicate for which `argN` must return `#t`.
`…music…`	normal LilyPond input, using `$` (in places where only LilyPond constructs are allowed) or `#` (to use it as a Scheme value or music function argument or music inside of music lists) to reference arguments (eg. ‘`#arg1`’).

The list of type predicates is required. Some of the most common type predicates used in music functions are:

boolean?
cheap-list?  (use instead of ‘list?’ for faster processing)
ly:duration?
ly:music?
ly:pitch?
markup?
number?
pair?
string?
symbol?

For a list of available type predicates, see Predefined type predicates. User-defined type predicates are also allowed.