New read/eval Scheme syntax inconsistent in handling existing code

[Valentin Villenave]

Greetings everybody,

David's new code with #/$(scheme) styles does offer a lot of exciting
possibilities... but it is also hugely disruptive when it comes to
existing code! Consider for instance the following:

\version "2.15.19"

myvar = { b'4 }

#(ly:parser-include-string parser
  "myvar = { a'2 }")

\new Staff \myvar

This used to print the overwritten variable, which it doesn't anymore
and I think I get why.

What's a lot more baffling, however, is that the variable isn't
overwritten _even_ when prefixing the Scheme expression with $ instead
of #.
(At least it isn't overwritten until after Lily's already parsed and
processed the staff.)

Thoughts?

Valentin.