reserve a,b,c,d,x,y,z for object, X,Y,Z for set;
reserve R,S,T for Relation;
reserve F,G for Function;

theorem Th11:
  R |_2 X = X|`(R|X)
proof
  thus R |_2 X = X|`R|X by Th10
    .= X|`(R|X) by RELAT_1:109;
end;
