  A version of the situation calculus in which situations are represented as
first-order terms is presented. Fluents can be computed from the term
structure, and actions on the situations correspond to rewrite rules on the
terms. Actions that only depend on or influence a subset of the fluents can be
described as rewrite rules that operate on subterms of the terms in some cases.
If actions are bidirectional then efficient completion methods can be used to
solve planning problems. This representation for situations and actions is most
similar to the fluent calculus of Thielscher \cite{Thielscher98}, except that
this representation is more flexible and more use is made of the subterm
structure. Some examples are given, and a few general methods for constructing
such sets of rewrite rules are presented. This paper was submitted to FSCD 2020
on December 23, 2019.
