SchurMultiplier(chr)
chr must be a cohomology-record that was created by a call of
CHR(G,p,[F],[mats]). SchurMultiplier calculates
the p-part Mul_p of the Schur multiplier Mul of G.
The result is returned as a list of integers, which are the
abelian invariants of Mul_p. If the list is empty, then
Mul_p is trivial.
GAP 3.4.4