chr must be a cohomology-record, created by a call of
CHR(G,p,F,[mats]), where F is a finitely presented group.
CoveringGroup calculates a presentation of a covering extension of Mul_p
by G, where Mul_p is the p-part of the Schur multiplier Mul of G.
The set of generators of the finitely presented group that is returned
is a union of two sets, which are in one-one correspondence with the
generators of F and of Mul_p, respectively.
The relators fall into three classes:
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