How do I find the arity of an Expression? (dmd hacking)

[Chad J]

Given an Expression object in dmd, I'd like to know how many
subexpressions it contains and, even better, iterate over them.  I'd
like to do this in a general way, without having to create cases for all
of the different kinds of Expressions.  Is there some way to do this
that I've been missing?

Thanks,
- Chad