A second idiom of Prolog is the use of assignment. Assignment is related to unification in that many variables, especially those in predicate calculus form, are assigned values through unification....

A second idiom of Prolog is the use of assignment. Assignment is related to unification in that many variables, especially those in predicate calculus form, are assigned values through unification. However, when a variable is to have some value based on an explicit functional calculation, the is operator must be used. Understanding the specific roles of assignment, evaluation, and pattern matching is important for Prolog use.