AgGroupOps.IsSubgroup( G, U )
If G is a parent group of U, then AgGroupOps.IsSubgroup returns
true. If the CGS of U is longer than that of G, U cannot be a
subgroup of G. Otherwise AgGroupOps.IsSubgroup shifts each generator
of U through G (see SiftedAgWord) in order to check if U is a
subgroup of G.
GAP 3.4.4