User:Dfeuer/Definition:Cone Compatible with Operation

Definition
Let $(S,\circ)$ be a semigroup.

Let $C$ be a subset of $S$.

Suppose that for each $x,y \in S$:
 * If $x, y \in C$ then $x \circ y \in C$
 * If $x \circ y \in C$ then $y \circ x \in C$.

Then $C$ is a cone compatible with $\circ$.