7.19. Point $P$ moves along the circumcircle of square $A B C D$. Lines $A P$ and $B D$ intersect at point $Q$, and the line through point $Q$ parallel to $A C$ intersects line $B P$ at point $X$. Find the locus of point $X$.
7.19. Points $P$ and $Q$ lie on the circle with diameter $D X$, so $\angle(Q D, D X)=\angle(Q P, P X)=\angle(A P, P B)=45^{\circ}$, i.e., point $X$ lies on the line $C D$.