Let $\mathscr{A}$ be a category. A map $X \stackrel{f}{\longrightarrow} Y$ in $\mathscr{A}$ is epic (or an epimorphism) if for all objects $Z$ and maps $\parpairi{Y}{Z}{g}{g’}$, \(g \circ f = g' \circ f \implies g = g'.\)