Feature request \includeonce

[Urs Liska]

Hi,

I'm missing a command in LilyPond to include a file only once.
I am working on a general purpose LilyPond library where I am in the 
situation that many library files have to access common functions and 
therefore have to include the same file (with these helper functions).
Although I assume that it doesn't harm to include a file several times 
(I think the functions will just be redefined identically) this looks 
like quite undesired behaviour.

I could figure out a Scheme based solution myself and also found some 
references to threads on lilypond-user (mainly from 2010). Basically the 
idea would be to define a variable in that helper file and subsequently 
only include that file if that variable isn't defined already.
But I think that functionality is quite fundamental and should be 
supported by LilyPond directly.

Therefore I would like to ask for the following feature:

"Implement a command \includeonce that includes a file just like 
\include, but only once.
Like include_once() in PHP or \RequirePackage in LaTeX."

Best
Urs