Definition. The \mathit {apply} function [dt-002N]

The \mathit {apply} function simply applies a given function to a given argument: \begin {array}{l} \mathit {apply} : ((A \twoheadrightarrow B) \times A) \twoheadrightarrow B\\ \mathit {apply} (f, a) = f(a) \end {array}