
theorem Th7:
  for X being set, x, y being object, T being Tolerance of X
  st x in Class (T, y) holds y in Class (T, x)
proof
  let X be set, x, y be object, T be Tolerance of X;
  assume x in Class (T, y);
  then [x,y] in T by EQREL_1:19;
  then [y,x] in T by EQREL_1:6;
  hence thesis by EQREL_1:19;
end;
