My latest Lilypond project had been compiling satisfactorily. But at a
certain point it began throwing out the following message:
%%%%%
Preprocessing graphical
objects.../usr/local/lilypond/usr/share/lilypond/current/ly/engraver-
init.ly:836:5: In procedure ly:stencil-extent in _expression_
(ly:stencil-extent (ly:note-head::print g) X):
This particular line is trying to find the X-extent (horizontal extent) of the note-head you are printing.
X and Y are defined as Scheme constants for the X (horizontal) and Y (vertical) directions.
One feature of Scheme is that you can redefine anything. So you redefined X and it caused problems.
A general rule for lilypond is to avoid any variable names that are all uppercase. In addition to X and Y, LEFT, CENTER, RIGHT, UP, and DOWN are all defined Scheme constants, and if you redefine them it will cause problems.
HTH,
Carl