let A be QC-alphabet ; :: thesis: for p, q, r, s, t being Element of CQC-WFF A holds (t => ((p => r) => s)) => ((p => q) => (t => ((q => r) => s))) in TAUT A

let p, q, r, s, t be Element of CQC-WFF A; :: thesis: (t => ((p => r) => s)) => ((p => q) => (t => ((q => r) => s))) in TAUT A

( (p => q) => (((p => r) => s) => ((q => r) => s)) in TAUT A & ((p => q) => (((p => r) => s) => ((q => r) => s))) => ((t => ((p => r) => s)) => ((p => q) => (t => ((q => r) => s)))) in TAUT A ) by Lm2, Lm3;

hence (t => ((p => r) => s)) => ((p => q) => (t => ((q => r) => s))) in TAUT A by CQC_THE1:46; :: thesis: verum

let p, q, r, s, t be Element of CQC-WFF A; :: thesis: (t => ((p => r) => s)) => ((p => q) => (t => ((q => r) => s))) in TAUT A

( (p => q) => (((p => r) => s) => ((q => r) => s)) in TAUT A & ((p => q) => (((p => r) => s) => ((q => r) => s))) => ((t => ((p => r) => s)) => ((p => q) => (t => ((q => r) => s)))) in TAUT A ) by Lm2, Lm3;

hence (t => ((p => r) => s)) => ((p => q) => (t => ((q => r) => s))) in TAUT A by CQC_THE1:46; :: thesis: verum