Suppose that the following result is known:
"Let $G$ be a finite set, closed with respect to an associative product and that both of the cancellation laws are valid. Then $G$ is a group with respect to this product."
The question is: "Find a counter-example which shows that, if one supposes only one of the cancellation laws, then we can't to conclude that $G$ is a group."
Cancellation laws: $$ax=ay \implies x=y$$ $$xa=ya \implies x=y$$
Which is this counter-example?! I can't find it!! Need some help...