Removes all the visible vertices that a given face is able to see which are stored in the 'assigned' vertext list
function(face) {
      if (face.outside !== null) {
        // reference to the first and last vertex of this face

        var start = face.outside;
        var end = face.outside;

        while (end.next !== null && end.next.face === face) {
          end = end.next;
        }

        this.assigned.removeSubList(start, end);

        // fix references

        start.prev = end.next = null;
        face.outside = null;

        return start;
      }
    }