Let rVabelemJmx v x : x \in G -> rV_E (v *m r x) = (rV_E v) ^ x.
Fact abelem_mx_repr : mx_repr G r.
Canonical abelem_repr := MxRepresentation abelem_mx_repr.
Let rG := abelem_repr.

Lemma abelem_rowgJ m (A : 'M_(m, n)) x :
x \in G -> rV_E @* rowg (A *m rG x) = (rV_E @* rowg A) :^ x.