reserve u,v,x,x1,x2,y,y1,y2,z,p,a for object,
        A,B,X,X1,X2,X3,X4,Y,Y1,Y2,Z,N,M for set;
reserve e for object, X,X1,X2,Y1,Y2 for set;

theorem
  for x being set, X being trivial set st x in X holds X = {x}
proof
  let x be set, X be trivial set;
  assume
A1: x in X;
  then ex x being object st X = {x} by Th130;
  hence thesis by A1,TARSKI:def 1;
end;
