let X be Subset of MC-wff; :: thesis: for p, q, r being Element of MC-wff holds (p => r) => ((q => r) => ((p 'or' q) => r)) in CnIPC X
let p, q, r be Element of MC-wff ; :: thesis: (p => r) => ((q => r) => ((p 'or' q) => r)) in CnIPC X
for T being Subset of MC-wff st T is IPC_theory & X c= T holds
(p => r) => ((q => r) => ((p 'or' q) => r)) in T ;
hence (p => r) => ((q => r) => ((p 'or' q) => r)) in CnIPC X by Def15; :: thesis: verum