The problem happens when the variable "function"
attempts to look up the procedure for 'control-points.
Each override you do is tacked onto the front of the
alist and so the key we're looking for-- 'control-points
-- is pushed further back. You can see what I mean by
changing cdr in the assignment for function (line 20?) to
cddr for one override, cdddr for two overrides... This
will "fix" the problem--provided the overrides come after
\shape ... (The reason why cdr is used in the first place
is that the current override of 'control-points would be
there at the head of the list, and you'd get
"calculation-in-progress" instead of
ly:slur::calc-control-points or whatever.) I'll think
about how to fix this and get back to you.
Thank you for noticing this!!