Intersection with Complement is Empty iff Subset

Theorem

 * $S \subseteq T \iff S \cap \complement \left({T}\right) = \varnothing$

where:
 * $S \subseteq T$ denotes that $S$ is a subset of $T$
 * $S \cap T$ denotes the intersection of $S$ and $T$
 * $\varnothing$ denotes the empty set
 * $\complement$ denotes set complement.

Also see

 * Complement Union with Superset is Universe