A set enumerated by some enumerator is said to be recursively enumerable. In 1850 he served as a census enumerator for Harrison County. In 1815 Pennington served as the Indiana Territory's census enumerator. The enumerator names are usually identifiers that behave as constants in the language. The remaining 30% received the questionnaire from an enumerator, as in previous censuses. An enumerator formula 1 is usually represented as a 2-tape Turing machine. The type can be a reference or an enumerator. Some enumerator types may be built into the language. Ruby supports generators (starting from version 1.9) in the form of the built-in Enumerator class.