 Thus each formula_11 set has at least one index, which describes the computable function enumerating the basic open sets from which it is composed; in fact it will have infinitely many such indices.Similarly, an index for a formula_14 set "B" describes the computable function enumerating the basic open sets in the complement of "B".A set "A" is lightface formula_18 if it is a union of a computable sequence of formula_14 sets (that is, there is a computable enumeration of indices of formula_14 sets such that "A" is the union of these sets).