let p, q be Element of HP-WFF ; :: thesis: (p => (p => q)) => (p => q) in HP_TAUT
(p => (p => q)) => ((p => p) => (p => q)) in HP_TAUT by Def10;
then A1: (p => p) => ((p => (p => q)) => (p => q)) in HP_TAUT by Th20;
p => p in HP_TAUT by Th14;
hence (p => (p => q)) => (p => q) in HP_TAUT by A1, Def10; :: thesis: verum