Commutative operation

A commutative function $f$ is a function that takes multiple inputs from a set $X$ and produces an output that does not depend on the ordering of the inputs. For example, the binary operation $+$ is commutative, because $3+4=4+3.$ The string concatenation function concat is not commutative, because $\mathrm{concat}(\text{"3"},\text{"4"})=\text{"34"}\ne \mathrm{concat}(\text{"4"},\text{"3"})=\text{"43"}$.