the field q("t") has a unique structure as a differential field, determined by setting ∂("t") = 1: the field axioms along with the axioms for derivations ensure that the derivation is differentiation with respect to "t". for example, by commutativity of multiplication and the leibniz law one has that ∂("u") = "u" ∂("u") + ∂("u")"u" = 2"u"∂("u").