can't subs(lst) on a function

[Chris Dams]

Hello Hugo,

I know casting to an ex is a solution (would not call it ``the solution''
though: ``the solution'' is editing the source files and recompiling
GiNaC). It is not natural that you can use the method subs(lst) to an
expression but not to a function. So, unless it turns out to be some weird
compiler error in my gcc-version, I would rather see this fixed in future
GiNaC versions than condemning future generations to having to use this
cast.

Greetings,
Chris