GAP 4.8.9 installation with standard packages -- copy to your CoCalc project to get it
LoadPackage( "RingsForHomalg" ); R2 := HomalgRingOfIntegersInSingular( 2 ) * "x,y,z"; R5 := HomalgRingOfIntegersInSingular( 5, R2 ) * "x,y,z"; m5 := HomalgMatrix( "[ 2 ]", 1, 1, R5 ); Assert( 0, IsZero( m5 ) = false ); m2 := R2 * m5; Assert( 0, IsZero( m2 ) );