reserve X,Y,Z,x,y,z for set;
reserve T,R for Tolerance of X;

theorem Th21:
  for x st x in X ex Z being TolClass of T st x in Z
proof
  let x;
  assume x in X;
  then [x,x] in T by Th7;
  then ex Z being TolClass of T st x in Z & x in Z by Th20;
  hence thesis;
end;
