
theorem Th43:
  for X being Subset of I(01), a, b being Point of I[01] st X = ].
  a, b .[ holds X is open
proof
  let X be Subset of I(01), a, b be Point of I[01];
  assume
A1: X = ]. a, b .[;
  then reconsider X9 = X as Subset of I[01] by Th16;
  X9 is open by A1,Th42;
  hence thesis by TSEP_1:17;
end;
