let p, q, r, s be Element of MC-wff ; :: thesis: (((q => r) => (p => r)) => s) => ((p => q) => s) in IPC-Taut
(p => q) => ((q => r) => (p => r)) in IPC-Taut by Th24;
hence (((q => r) => (p => r)) => s) => ((p => q) => s) in IPC-Taut by Th25; :: thesis: verum