I've found this especially useful in combination with my own
_attr_uint_const for things like flags and type specifiers.

It's possible that its usefulness might be diminished significantly if
there were a built-in bitfield constant parser -- that certainly would
eliminate all of my current uses of it -- but it still seems nicely
symmetric with H_ACT_APPLY.

Tabs after the first non-tab character are crazymaking. I picked 8
because it wasn't 7 and caused the backslashes to line up on
H_ACT_APPLY.