Description: Define the equivalence relation in a group generated by a subgroup.
More precisely, if G is a group and H is a subgroup, then
G ~QG H is the equivalence relation on G associated with the
left cosets of H . A typical application of this definition is the
construction of the quotient group (resp. ring) of a group (resp. ring)
by a normal subgroup (resp. two-sided ideal). (Contributed by Mario
Carneiro, 15-Jun-2015)