Let ABC be a right angled triangle with ∠B=900 and let BD be the altitude from B on to AC. Draw DE⊥AB and DF⊥BC. Let P, Q, R and S be respectively the incenters of triangle DFC, DBF, DEB and DAE. Suppose S, R, Q are collinear. Prove that P, Q, R, D lie on a circle. (RMO 2015, Mumbai)