In working with functions of
Baire class one having the Darboux property, one of the most useful tools has been a
theorem due to Baire that says a function of Baire class one has a point of continuity
on every closed set relative to the closed set. The lemma mentioned in the title can
be used in many instances more efficiently than Baire’s theorem as is shown in §4. It
is concerned with sets rather than functions and hence more basic than Baire’s
Theorem, and easier to prove requiring only one application of Baire’s category
theorem.