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