Let $ABCD$ be a cyclic quadrilateral. Let $P$ be the intersection of $\overleftrightarrow{AD}$ and $\overleftrightarrow{BC}$, and let $Q$ be the intersection of $\overleftrightarrow{AB}$ and $\overleftrightarrow{CD}$. Prove that the angle bisectors of $\angle DPC$ and $\angle AQD$ are perpendicular.