
theorem Th21:
  for T being non empty TopSpace, p being Point of T for A being
  Element of OpenNeighborhoods p holds A is a_neighborhood of p
proof
  let T be non empty TopSpace, p be Point of T, A be Element of
  OpenNeighborhoods p;
  ex W being Subset of T st W = A & p in W & W is open by YELLOW_6:29;
  hence thesis by CONNSP_2:3;
end;
