Symmetry Group is Group

Theorem
Let $P$ be a geometric figure.

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

Let $\circ$ denote composition of mappings.

The symmetry group $\left({S_P, \circ}\right)$ is indeed a group.

Proof
As a symmetry is a bijection, we can use the result Group of Permutations is a Group.