 The analogous formula_11 sets, with a lightface formula_12, are no longer "arbitrary" unions of such neighborhoods, but computable unions of them.That is, a set is lightface formula_11, also called "effectively open", if there is a computable set "S" of finite sequences of naturals such that the given set is the union of the sets {"x"∈ωω|"s" is an initial segment of "x"} for "s" in "S".A set is lightface formula_14 if it is the complement of a formula_11 set.