let Al be QC-alphabet ; :: thesis: for p, q being Element of CQC-WFF Al holds p => (('not' p) => q) is valid
let p, q be Element of CQC-WFF Al; :: thesis: p => (('not' p) => q) is valid
p => (('not' p) => q) in TAUT Al

proof

TAUT Al is being_a_theory by Th15;
hence p => (('not' p) => q) in TAUT Al by Def1; :: thesis: verum

end;

hence p => (('not' p) => q) is valid by Lm13; :: thesis: verum