Possible issue in substitution trees - check whether _nextVar really needs to keep growing forever

[quickbeam123]

(The problem being we would eventually run out _nextVars - which actually happened before, in an actual run, when SPEC_UPPER_BOUND was "only" 10000000, now it's about 50times larger.)

From Michael:

"We only "run out" of special variables because substitution trees do something a bit surprising with _nextVar to achieve freshness."

[MichaelRawson]

@ibnyusuf @joe-hauns - I believe this can now be closed and SPEC_UPPER_BOUND can be deleted. We no longer have "very special vars" so all special variables are just the right amount of special. What do you think?

[joe-hauns]

Yesss I think you're right. I think i just didn't spot that when i removed very special variables.

[MichaelRawson]

Thanks! I'll add it to the default-constructor PR.