Definitions and sentences are called forms . A knowledge base is a finite set of forms. Individual variables, constants, character references, strings, and blocks were discussed earlier. A functional term consists of a function constant and an arbitrary number of argument terms, terminated by an optional sequence variable and surrounded by matching parentheses. A list term consists of the listof operator and a finite list of terms, terminated by an optional sequence variable and enclosed in matching parentheses.
Quotations involve the quote operator and an arbitrary list expression .
A list expression is either an atom or a sequence of list expressions surrounded by parentheses. An atom is either a word or a character reference or a character string or a character block. Logical terms involve the if and cond operators. The if form allows for the testing of a single condition or multiple conditions. An optional term at the end allows for the specification of a default value when all of the conditions are false.