theorem Th79:
  X1 is open SubSpace of X0 implies for x0 being Point of X0, x1
  being Point of X1 st x0 = x1 holds g is_continuous_at x0 iff g|X1
  is_continuous_at x1
proof
  assume
A1: X1 is open SubSpace of X0;
  let x0 be Point of X0, x1 be Point of X1;
  assume
A2: x0 = x1;
  hence g is_continuous_at x0 implies g|X1 is_continuous_at x1 by A1,Th74;
  thus g|X1 is_continuous_at x1 implies g is_continuous_at x0
  proof
    reconsider A = the carrier of X1 as Subset of X0 by A1,TSEP_1:1;
    assume
A3: g|X1 is_continuous_at x1;
    A is open by A1,TSEP_1:16;
    hence thesis by A1,A2,A3,Th77;
  end;
end;
