[GiNaC-devel] calchash and compare_same_type for symmetries.

[Chris Dams]

Dear all,

The commentary of compare_same_type used to say that "All symmetry trees
are equal. They are not supposed to appear in ordinary expressions
anyway...". This is wrong, however, because for archiving purposes we need 
to know that whether symmetry trees are equal or not. The reason is that 
any particular expression occurs only once in an archive for reasons of 
conserving space.

I fixed this by adding a calchash method to the class symmetry and by 
making compare_same_type do the right thing.

Best wishes,
Chris