If $A,B \subseteq X$ and $|N(A)|=|A|$ and $|N(B)|=|B|$ in a finite bipartite graph $G$ with bipartition $X\cup Y$ then is it true that if Hall's condition holds on $X$ that:
$|N(A \cup B)|=|A \cup B|$ ?
It seems very obvious in my head, visually, but I don't know how to show it.