
theorem Th2:
  for M being PseudoMetricSpace,x,y being Element of M holds y in x
  -neighbour iff y tolerates x
proof
  let M be PseudoMetricSpace,x,y be Element of M;
  hereby
    assume y in x-neighbour;
    then ex q be Element of M st y = q & x tolerates q;
    hence y tolerates x;
  end;
  assume y tolerates x;
  hence thesis;
end;
