Containment deals with subset and superset relationships. If $A \subseteq B$, then $A$ is a subset of $B$ meaning that all elements of $A$ are also members of $B$. If $A \subset B$, then $A$ is a proper subset of $B$ which is a subset such that $A \neq B$. The reversal of these relationships are superset, $B \supseteq A$, and proper superset, $B \supset A$, respectively.
import proveit %theory