PGMapCSS is a library for PostgreSQL/PostGIS which works between an osm2pgsql based database and Mapnik (and maybe other renderers). It processes database (usually OpenStreetMap) objects according to MapCSS rules and calculates resulting colors, widths and other properties for Symbolizers, even geometric modifications.
Sometimes, some mapnik rules will not be produced in output. Fixed in this pull request.
Problem was, that prop() may always return
none. This sometimes happened with eval() statements.