let p, q, r be Element of MC-wff ; :: thesis: (p => r) => ((q => r) => ((p 'or' q) => r)) is valid_IPC
(p => r) => ((q => r) => ((p 'or' q) => r)) in IPC-Taut

proof

IPC-Taut is IPC_theory ;
hence (p => r) => ((q => r) => ((p 'or' q) => r)) in IPC-Taut ; :: thesis: verum

end;

hence (p => r) => ((q => r) => ((p 'or' q) => r)) is valid_IPC ; :: thesis: verum