subgroup.lean : general subgroup theories, quotient group using quot
finsubg.lean : finite subgroups (finset and fintype), Lagrange theorem,
finite cosets and lcoset_type, normalizer for finite groups, coset product
and quotient group based on lcoset_type, semidirect product
hom.lean : homomorphism and isomorphism, kernel, first isomorphism theorem
perm.lean : permutation group
cyclic.lean : cyclic subgroup, finite generator, order of generator, sequence and rotation
action.lean : fixed point, action, stabilizer, orbit stabilizer theorem, orbit partition,
Cayley theorem, action on lcoset, cardinality of permutation group
pgroup.lean : subgroup with order of prime power, Cauchy theorem, first Sylow theorem
