let X be Subset of CQC-WFF ; :: thesis: for p, q being Element of CQC-WFF holds p => (('not' p) => q) in Cn X
let p, q be Element of CQC-WFF ; :: thesis: p => (('not' p) => q) in Cn X
A1: for T being Subset of CQC-WFF st T is being_a_theory & X c= T holds
p => (('not' p) => q) in T by Def1;
thus p => (('not' p) => q) in Cn X by A1, Def2; :: thesis: verum