let L be with_infima Poset; :: thesis: for x, y being Element of (L opp) holds (~ x) "/\" (~ y) = x "\/" y
let x, y be Element of (L opp); :: thesis: (~ x) "/\" (~ y) = x "\/" y
( (~ x) ~ = ~ x & (~ y) ~ = ~ y ) ;
hence (~ x) "/\" (~ y) = x "\/" y by Th21; :: thesis: verum