In structured KIF, the notion of word is taken as primitive. An expression is either a word or a finite sequence of expressions. The set of all words is divided into the categories listed below. This categorization is disjoint and exhaustive.
Every word is a member of one and only one category.
Individual variables are used in quantifying over individual objects. Sequence variables are used in quantifying over sequences of objects. Operators are used in forming complex expressions of various sorts. There are four types of operators in KIF - term operators , sentence operators , rule operators , and definition operators . Term operators are used in forming complex terms.