\begin{definition}[Definition:Opposite Magma]
Let $S$ be a set.
Let $\struct {S, \circ}$ and $\struct {S, *}$ be magmas on $S$.
$\struct {S, *}$ is the '''opposite magma of $\struct {S, \circ}$''' {{iff}}:
:$\forall x_1, x_2, x_3 \in S: x_1 \circ x_2 = x_3 \iff x_2 * x_1 = x_3$
The operation $*$ is sometimes referred to as the '''opposite law''' of $\circ$.
\end{definition}