Extending LilyPond: 2.1.3 Intermediate substitution functions

### 2.1.3 Intermediate substitution functions

Intermediate substitution functions involve a mix of Scheme code and LilyPond code in the music expression to be returned.

Some `\override` commands require an argument consisting of a pair of numbers (called a cons cell in Scheme).

The pair can be directly passed into the music function, using a `pair?` variable:

>     manualBeam =
>     #(define-music-function
>          (parser location beg-end)
>          (pair?)
>        #{
>          \once \override Beam #'positions = $beg-end
>        #})

>     \relative c' {
>       \manualBeam #'(3 . 6) c8 d e f
>     }

Alternatively, the numbers making up the pair can be passed as separate arguments, and the Scheme code used to create the pair can be included in the music expression:

>     manualBeam =
>     #(define-music-function
>          (parser location beg end)
>          (number? number?)
>        #{
>          \once \override Beam #'positions = $(cons beg end)
>        #})

>     \relative c' {
>       \manualBeam #3 #6 c8 d e f
>     }

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#### LilyPond — Extending