let Y be non empty set ; :: thesis: for a, c, b being Function of Y,BOOLEAN holds (a 'imp' c) '&' (b 'imp' c) '<' (a 'or' b) 'imp' c
let a, c, b be Function of Y,BOOLEAN; :: thesis: (a 'imp' c) '&' (b 'imp' c) '<' (a 'or' b) 'imp' c
((a 'imp' c) '&' (b 'imp' c)) 'imp' ((a 'or' b) 'imp' c) = I_el Y by BVFUNC_6:9;
hence (a 'imp' c) '&' (b 'imp' c) '<' (a 'or' b) 'imp' c by BVFUNC_1:16; :: thesis: verum