let L be B_Lattice; :: thesis: for X, Y being Element of L holds (X \ Y) ` = (X `) "\/" Y
let X, Y be Element of L; :: thesis: (X \ Y) ` = (X `) "\/" Y
(X \ Y) ` = (X `) "\/" ((Y `) `) by LATTICES:23;
hence (X \ Y) ` = (X `) "\/" Y ; :: thesis: verum