GAP 4.8.9 installation with standard packages -- copy to your CoCalc project to get it
gap> START_TEST("HAP library"); gap> 2simplices:= [[1,2,4], [2,4,8], [2,3,8], [3,8,9], [1,3,9], [1,4,9], [4,5,8], [5,6,8], [6,8,9], [6,7,9], [4,7,9], [4,5,7], [1,5,6], [1,2,6], [2,6,7], [2,3,7], [3,5,7], [1,3,5]];; gap> T:=RegularCWComplex(SimplicialComplex(2simplices));; gap> D:=DiagonalApproximation(T);; gap> p:=D.projection;; gap> P:=ChainMap(p);; gap> IsIsomorphismOfAbelianFpGroups(Homology(P,0)); true gap> IsIsomorphismOfAbelianFpGroups(Homology(P,1)); true gap> IsIsomorphismOfAbelianFpGroups(Homology(P,2)); true gap> Size(Source(p)); 180 gap> Size(Source(P)); 4 gap> Size(Target(p)); 54 gap> Size(Target(P)); 4 gap> del:=D.inclusion;; gap> Del:=ChainMap(del);; gap> Size(Target(Del)); 16 gap> STOP_TEST( "tst.tst", 1000 );