theorem Th130:
 X is non empty trivial implies ex x st X = {x}
 proof
  assume X is non empty;
   then consider x being object such that
A1:  x in X;
  assume
A2:  X is trivial;
  take x;
  for y being object holds y in X iff x = y by A2,A1;
  hence thesis by TARSKI:def 1;
 end;
