%destructor declared by semantic type

[Joel E. Denny]

I'd like to start working on making this possible:

  %destructor { free ($$); } symbol1 <type1> <type2>

This would declare the same destructor for the symbol `symbol1' and for 
all symbols declared with semantic type `type1' or `type2'.

One benefit that may not be immediately obvious is that Bison would then 
be able to compute which destructor to use for a midrule semantic value.  
However, this raises questions about how $<type>$ should be handled in 
general.

I think I've come up with a reasonable plan to handle all this.  My full 
proposal is here:

  http://lists.gnu.org/archive/html/bison-patches/2006-02/msg00067.html

What do you think?

Joel