plus0xxplussxysplusxytimes0y0timessxyplusytimesxypsxxp00minusx0xminus0x0minusxsypminusxyisZero0trueisZerosxfalsefacIterxyifisZeroxminusxs0ytimesyxiftruexyzyiffalsexyzfacIterxzfactorialxfacIterxs0plus200s1times2p1minus2isZero1true0false0facIter2if4factorial1INNERMOST./TRS/Thiemann/factorial2.trs