let p, q, s be Element of HP-WFF ; :: thesis: (q => s) => ((p '&' q) => s) in HP_TAUT
set P = p '&' q;
A1: (p '&' q) => q in HP_TAUT by Def10;
((p '&' q) => q) => ((q => s) => ((p '&' q) => s)) in HP_TAUT by Th21;
hence (q => s) => ((p '&' q) => s) in HP_TAUT by A1, Def10; :: thesis: verum