gap> K:=BarycentricSubdivision(ContractibleGcomplex("SL(2,Z)"));;
gap> G:=HAP_CongruenceSubgroupGamma0(2^17-1);;
gap> Y:=GComplexToRegularCWComplex(K,G);
Regular CW-complex of dimension 1

gap> Length(Homology(Y,1));
21845
