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

[Ary Borenszweig]

Chad J wrote:
> 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

Where's the Expression object defined in D? I don't think there's such a 
thing (I mean, a reflection capability to get an expression out of 
something).