let p, q, r be Element of HP-WFF ; :: thesis: (p => q) => ((q => r) => (p => r)) in HP_TAUT
(q => r) => ((p => q) => (p => r)) in HP_TAUT by Th19;
hence (p => q) => ((q => r) => (p => r)) in HP_TAUT by Th20; :: thesis: verum