A simple frame-based rules engine that supports both forward and backward chaining.
Marvin Rules is distributed under the Apache V2 license (http://www.apache.org/licenses/LICENSE-2.0.html).
The source code of Marvin Rules can be found on https://github.com/paulhh/marvin-rules.
Loading marvin.pl will include all the necessary code.
The code has been created for use with the tuProlog (Java-based) engine, but should be easy to adjust for other Prologs. tuProlog can be found here.
Let's just say it's a 0.1 version.