Enumeration operators are of central importance in the study of enumeration reducibility. For example, a PID namespace provides a separate enumeration of process identifiers within each namespace. C exposes the integer representation of enumeration values directly to the programmer. Historical censuses used crude enumeration assuming absolute accuracy. In English, there are grammatical rules for specifying coordinate items in an enumeration or series. The census of 1920 was the first national enumeration. Among census experts this method is called dual system enumeration (DSE). There are also more specific strategies like the continuous enumeration method that reproduces specific patterns. Random Search replaces the exhaustive enumeration of all combinations by selecting them randomly. A contiguous cartogram tries to retain the contiguity of enumeration units. They also allow class enumeration, cardinality, disjointness, and equivalence. Most commonly the conjoining coordination expresses enumeration, simultaneity or immediate sequence. The specific enumeration of rights has differed greatly in different periods of history. The Population enumeration schedule contained 30 questions. Let formula 14 be an enumeration of the vertices of formula 6.