I would like computing the rank of K/[K,K] where K is the kernel of a morphisme \psi defined as follow :

un:=Z(7)^0;
zero:=0*Z(7);

gk1:=[[1,1],[0,1]];
gk2:=[[1,0],[-E(3),1]];

gi1:=[[un, un], [zero, un]];
gi2:=[[un, zero], [2*un, un]];

GammaK:=Group([[1,1],[0,1]], [[1,0],[-E(3),1]]);
GammaI:=Group(gi1,gi2);

psi:=GroupHomomorphismByImages(GammaK,GammaI,
GammaK.generators,GammaI.generators);

Does a trick exist to avoid the instruction kernel(psi) ?