$A$ is a subset of $B$ iff every element of $A$ is also an element of $B$, denoted $A \subseteq B$
$$\forall x(x\in A \implies x \in B)$$$A$ is a subset of $B$ iff every element of $A$ is also an element of $B$, denoted $A \subseteq B$
$$\forall x(x\in A \implies x \in B)$$