PolygonSymbolizer ps = (PolygonSymbolizer) r.getSymbolizers()[0];
Expression geom = ps.getGeometry();
assertNotNull(geom);
assertTrue(geom instanceof FilterFunction_buffer);
FilterFunction_buffer buf = (FilterFunction_buffer) geom;
assertTrue(buf.getParameters().get(0) instanceof PropertyName);
assertTrue(buf.getParameters().get(1) instanceof Literal);
}