reserve A,X,X1,X2,Y,Y1,Y2 for set, a,b,c,d,x,y,z for object;
reserve P,P1,P2,Q,R,S for Relation;

theorem
  Y c= X implies Y|`(X|`R) = Y|`R
proof
  Y c= X implies Y /\ X = Y by XBOOLE_1:28;
  hence thesis by Th90;
end;
