Field defined as a field of fractions with a default implementation of GCD/LCM such that - gcd(a/b, c/d) = gcd(a, c) / lcm(b, d) - lcm(a/b, c/d) = lcm(a, c) / gcd(b, d) which corresponds to the convention of the GCD domains of SageMath; on rational numbers, it "yields the unique extension of gcd from integers to rationals presuming the natural extension of the divisibility relation from integers to rationals", see http://math.stackexchange.com/a/151431
Field with simple default GCD/LCM implementations: gcd(a, b) = 1 (except gcd(0, 0) = 0) while lcm(a, b) = a * b.