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

theorem
  R c= [:X,Y:] implies R~ c= [:Y,X:]
proof
  assume
A1: R c= [:X,Y:];
  let z,t be object;
  assume [z,t] in R~;
  then [t,z] in R by RELAT_1:def 7;
  then t in X & z in Y by A1,ZFMISC_1:87;
  hence thesis by ZFMISC_1:87;
end;
