Description: Sylow's second theorem. Any P -group H is a subgroup of a
conjugated P -group K of order P ^ n || ( #X ) with
n maximal. This is usually stated under the assumption that K
is a Sylow subgroup, but we use a slightly different definition, whose
equivalence to this one requires this theorem. This is part of Metamath
100 proof #72. (Contributed by Mario Carneiro, 18-Jan-2015)