An iterative characterization of computable functions on hyperwords of natural numbers

The set of words on a given alphabet is a well know data type which occurs in most of programming languages. The notion of word can be generalized to the notion of hyperword, by allowing that a word can be used as a new letter to construct new words. Hyperwords arise in various ways in computer science: e.g., consider the list structures of LISP-like languages or the operand stack of a PostScript interpreter [1].