CL-GEOS is a Common Lisp library for performing geometric operations. It allows you to create geometric objects (points, lines, rings, polygons, etc.) and easily query for intersections, area, centroid, and so on. Most of the heavy lifting is done by the excellent GEOS library.
Most of the GEOS API for I/O, predicates, and topology are implemented, however this library is still in beta. Every effort will be made to preserve the external API when bumping up to version 1.0.
Requires the GEOS library. On Debian based systems, this can be accomplished by running:
apt-get install libgeos-dev
The geos package exports
UNION which are two functions defined by CL. You'll want to shadow import these if you want the GEOS versions instead of the CL ones.