[anonymoushn]

For what value of __mull__ and __add__ does f(g(x)) produce a different result than h(x)? You could do this by making __mul__ or __add__ change behavior based on stack inspection, but I am having trouble thinking of another way.

[alextgordon]

A backtrace would be different, for a start. If __mul__ raises an exception, it would not show as coming from g(). Makes debugging a little bit harder.