let A be QC-alphabet ; :: thesis: for p, q, r, s being Element of CQC-WFF A holds (q => s) => ((s => (p => (q => r))) => (p => (q => r))) in TAUT A
let p, q, r, s be Element of CQC-WFF A; :: thesis: (q => s) => ((s => (p => (q => r))) => (p => (q => r))) in TAUT A
( (q => r) => (p => (q => r)) in TAUT A & ((q => r) => (p => (q => r))) => ((q => s) => ((s => (p => (q => r))) => (p => (q => r)))) in TAUT A ) by Lm20, Th5;
hence (q => s) => ((s => (p => (q => r))) => (p => (q => r))) in TAUT A by CQC_THE1:46; :: thesis: verum