An Expression tree walker that will visit each Expression e in the tree,
in depth-first evaluation order, and call fp(e,param) on it.
fp() signals whether the walking continues with its return value:
0 continue
1 done
It's a bit slower than using virtual functions, but more encapsulated and less brittle.
Creating an iterator for this would be much more complex.
An Expression tree walker that will visit each Expression e in the tree, in depth-first evaluation order, and call fp(e,param) on it. fp() signals whether the walking continues with its return value: