for X, Y being non empty TopSpace
for X0, X1 being non empty SubSpace of X
for g being Function of X0,Y st X1 is open SubSpace of X0 holds
for x0 being Point of X0
for x1 being Point of X1 st x0 = x1 holds
( g is_continuous_at x0 iff g | X1 is_continuous_at x1 )