Likely a good frog project for someone with C knowledge

[David Kastrup]

Compile Lilypond with
-DSCM_DEBUG_TYPING_STRICTNESS=2

The guile documentation states quite clearly

 -- C Type: SCM
     `SCM' is the user level abstract C type that is used to represent
     all of Guile's Scheme objects, no matter what the Scheme object
     type is.  No C operation except assignment is guaranteed to work
     with variables of type `SCM', so you should only use macros and
     functions to work with `SCM' values.  Values are converted between
     C data types and the `SCM' type with utility functions and macros.
 
Now the Lilypond code base is chock full of code comparing SCM values.
And it likely also contains a number of cases where C types (like
booleans) and Scheme type are getting mixed up.

Compiling with the above definition will make the compiler complain
about anything but assignments.

But with the current code base, cleaning this all up is going to be a
lot of work.  Not hard to do, but a lot.

-- 
David Kastrup