This section describes an implementation of the query interpreter that performs inference in the logic language. The interpreter is a general problem solver, but has substantial limitations on the scale and type of problems it can solve. More sophisticated logical programming languages exist, but the construction of efficient inference procedures remains an active research topic in computer science.

The fundamental operation performed by the query interpreter is called unification. Unification is a general method of matching a query to a fact, each of which may contain variables. The query interpreter applies this operation repeatedly, first to match the original query to conclusions of facts, and then to match the hypotheses of facts to other conclusions in the database. In doing so, the query interpreter performs a search through the space of all facts related to a query. If it finds a way to support that query with an assignment of values to variables, it returns that assignment as a successful result.