Composition of Symmetries is Associative

Theorem
Let $P$ be a geometric figure.

Let $S_P$ be the set of all symmetries of $P$.

Let $\circ$ denote composition of mappings.

Let $\phi, \psi, \chi$ be symmetries of $P$.

Then:
 * $\paren {\phi \circ \psi} \circ \chi = \phi \circ \paren {\psi \circ \chi}$

That is, composition of symmetries is associative.

Proof
From Composition of Symmetries is Symmetry:
 * $\paren {\phi \circ \psi} \circ \chi$ is a symmetry

and:
 * $\phi \circ \paren {\psi \circ \chi}$ is a symmetry.

It follows from Composition of Mappings is Associative that:
 * $\paren {\phi \circ \psi} \circ \chi = \phi \circ \paren {\psi \circ \chi}$