The function "F" is called universal if the following property holds: for every computable function "f" of a single variable there is a string "w" such that for all "x", "F"("w" "x") = "f"("x"); here "w" "x" represents the concatenation of the two strings "w" and "x". This means that "F" can be used to simulate any computable function of one variable. Informally, "w" represents a "script" for the computable function "f", and "F" represents an "interpreter" that parses the script as a prefix of its input and then executes it on the remainder of input.