h(x) = g(f(x)) — Composing Functions With ∘ and Its Reverse

The composition operator $\circ$ takes two functions $f$ and $g$ and returns a new function $h(x)=(g\circ f)(x)=g(f(x))$

Reverse composition ($f\mapsto g$) applies the operation in opposite order.

Reference

• https://en.wikipedia.org/wiki/Function_composition