Dan Nelson noted the following in the chat:
6:39 Genesys interesting. my amd64 pikefarm 7.8 fails the following test: return equal(Gmp.mpz(13)->gcdext(19), ({1, 3, -2})); 6:40 Genesys it returns ({ 1, -16, 11 }), which seems to be a correct answer if I do the math