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
  field {[x,x]} = {x}
proof
  thus field {[x,x]} = {x,x} by Th11
    .= {x} by ENUMSET1:29;
end;
