let A be QC-alphabet ; :: thesis: for p, q, r, s being Element of CQC-WFF A holds (((r => p) => p) => s) => (((p => q) => r) => s) in TAUT A
let p, q, r, s be Element of CQC-WFF A; :: thesis: (((r => p) => p) => s) => (((p => q) => r) => s) in TAUT A
( ((p => q) => r) => ((r => p) => p) in TAUT A & (((p => q) => r) => ((r => p) => p)) => ((((r => p) => p) => s) => (((p => q) => r) => s)) in TAUT A ) by Lm16, Th1;
hence (((r => p) => p) => s) => (((p => q) => r) => s) in TAUT A by CQC_THE1:46; :: thesis: verum