Privileged arguments
====================

As a matter of convenience, there are several identifiers that
internal macros use. Chances are that if you use these names for other
things, you're gonna have a bad time.

In particular, these names, and the macros that use them, are:
- state:
    Used by a_new and company. Should be an HParseState*
- mm__:
    Used by h_new and h_free. Should be an HAllocator*