 reserve x,y,z,t for object,X,Y,Z,W for set;
 reserve R,S,T for Relation;

theorem
  R c= S implies R~ c= S~
proof
  assume R c= S;
  then R \/ S = S by XBOOLE_1:12;
  then (R~) \/ (S~) = S~ by RELAT_1:23;
  hence thesis by XBOOLE_1:7;
end;
