In theoretical computer science and formal language theory, a ranked alphabet is a pair of an ordinary alphabet F and a function Arity: F→
Nullary elements (of zero arity) are also called constants.
Terms built with unary symbols and constants can be considered as strings.
Higher arities lead to proper trees.
Contrariwise, cannot be a valid term, as the symbol f appears once as binary, and once as unary, which is illicit, as Arity must be a function.