ZA-SAMO 1999 Problem 2

$A,\ B,\ C$ and $D$ are points on a given straight line, in that order. Show how to construct a square $PQRS$ , with all of $P,\ Q,\ R$ and $S$ on the same side of $AD$ , such that $A,\ B,\ C$ and $D$ lie on $PQ,\ SR,\ QR$ and $PS$ produced respectively.