let p, q, r be Element of MC-wff ; :: thesis: (p => q) => ((q => r) => (p => r)) in IPC-Taut
(q => r) => ((p => q) => (p => r)) in IPC-Taut by Th22;
hence (p => q) => ((q => r) => (p => r)) in IPC-Taut by Th23; :: thesis: verum