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in the original mapnik/osm style, barrier=hedge polygons are filled not in the cartocss version.
in others.mss there is #barriers_area but project.mml does not use it, but uses .barriers for both lines and polygons, so the polygon-fill can simply move there...
Resolve the naming discrepancy for the area barriers layer. Fixes #40
Just as a side note - you can't move the polygon-fill rule into the .barriers section. If you did, then all barrier=hedge objects would get a fill, including those from the line-barriers layer - so any 3-node linear hedge would show up as a triangle, for example.