This is a combination of many commits:
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final. add public/private for core classes.
wip - big cleanup cosets.
index<>, cosets<>, and group<> all defined
transparent <void> overrides that operate directly on indices, not names
inheritance hierarchy means one can drop or change generator names
still missing path support.
wip - named cosets
fix lang test
wip - named generators
Move Group docstring away from specialization.
Remove tc::Gen; use Group<void>
remove tc::Rel
add thousands separators for benchmark limits
add generators list to tc::group
remove pair_map
