This reflects the fact that positions stored in matches are
absolute positions, while Position type is used in the
language as position relative to current position in the
sentence context. So mixing the two was confusing at the very least.
This also actually fits the most recent version of spec,
which no longer mentions Position, either.

It didn't like passing reference to something that could be treated as a value.

Variables is noncopyable but default copy constructor was apparently working in VS.

Each subsequent rule was reusing variables object, which wasn't good.
Parsing of sequences had to be changed not to end with semi,
and it wasn't actually passing on lexicons in the first place.