let p, q, r, s be Element of HP-WFF ; :: thesis: (((q => r) => (p => r)) => s) => ((p => q) => s) in HP_TAUT
(p => q) => ((q => r) => (p => r)) in HP_TAUT by Th21;
hence (((q => r) => (p => r)) => s) => ((p => q) => s) in HP_TAUT by Th22; :: thesis: verum