VN-TST 2001 Problem 2

In the plane let two circles be given which intersect at two points $A, B$ ; Let $PT$ be one of the two common tangent line of these circles ( $P, T$ are points of tangency). Tangents at $P$ and $T$ of the circumcircle of triangle $APT$ meet each other at $S$ . Let $H$ be a point symmetric to $B$ under $PT$ . Show that $A, S, H$ are collinear.