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 concat("3","4")="34" does not equal concat("4","3")="43".