Hashing structures.

[MACRAK at MIT-MC (Stavros M. Macrakis)]

I agree that the current SXHash is rather poor.  But all schemes based on
the recursion step R(a,x)+R(b,y) are bound to lose, since it is so often
true that R(a+b,x)=R(a,x)+R(b,x), where R(a,x) is a rotated x positions.
Equal rotations tend to happen through the tree.

Some algebraic insight is missing here.  Some sort of easily-computable
finite binary operator with few identities is needed.  Ideas?