docs: function(orthobasis) {
              var EPS = 1e-5;
              var cags = [];
              this.polygons.filter(function(p) {
                      // only return polys in plane, others may disturb result
                      return p.plane.normal.minus(orthobasis.plane.normal).lengthSquared() < EPS*EPS;
                  })
                  .map(function(polygon) {
                      var cag = polygon.projectToOrthoNormalBasis(orthobasis);
                      if (cag.sides.length > 0) {
                          cags.push(cag);
                      }
              });
              var result = new CAG().union(cags);
              return result;
          }