C++ PATCH: PR 20599 (variadic templates, take two) (1/4)

[Jason Merrill]

I'm finding the term "pack/unpack" rather mysterious; it makes it sound 
like you can use a PACK_UNPACK_* to either pack or unpack arguments, or 
both at the same time.  I think something like *_PACK_EXPANSION or 
_SUBST would be clearer.

Also, the definition seem to allow for using multiple packs in a single 
expansion, but the proposal doesn't cover that.

More soon.

Jason