let L be RelStr ; :: thesis: for x being set holds
( x is lower Subset of (L opp ) iff x is upper Subset of L )
let x be set ; :: thesis: ( x is lower Subset of (L opp ) iff x is upper Subset of L )
( x is upper Subset of L iff x is upper Subset of ((L opp ) ~ ) ) by WAYBEL_0:25;
hence ( x is lower Subset of (L opp ) iff x is upper Subset of L ) by Th28; :: thesis: verum