let Al be QC-alphabet ; :: thesis: for X being Subset of (CQC-WFF Al)
for p being Element of CQC-WFF Al holds (('not' p) => p) => p in Cn X
let X be Subset of (CQC-WFF Al); :: thesis: for p being Element of CQC-WFF Al holds (('not' p) => p) => p in Cn X
let p be Element of CQC-WFF Al; :: thesis: (('not' p) => p) => p in Cn X
for T being Subset of (CQC-WFF Al) st T is being_a_theory & X c= T holds
(('not' p) => p) => p in T ;
hence (('not' p) => p) => p in Cn X by Def2; :: thesis: verum