A function f:X→Yf: X \to Yf:X→Y, g:f(X)→Zg: f(X) \to Zg:f(X)→Z is defined as follows: the composition of ggg with fff is called h:X→Zh: X \to Zh:X→Z, and it is denoted by h=g∘fh=g \circ fh=g∘f.
h(x)=(g∘f)(x):=g(f(x)) h(x) = (g\circ f) (x) := g\left( f(x) \right) h(x)=(g∘f)(x):=g(f(x))