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 Th11:
  field {[x,y]} = {x,y}
proof
  dom {[x,y]} = {x} & rng {[x,y]} = {y} by Th3;
  hence thesis by ENUMSET1:1;
end;
