code: private void interiorAreaBoundaryArea_(int half_edge, int id_a,
  			int predicate) {
  		if (m_matrix[predicate] == 1)
  			return;
  
  		int faceParentage = m_topo_graph.getHalfEdgeFaceParentage(half_edge);
  		int twinFaceParentage = m_topo_graph
  				.getHalfEdgeFaceParentage(m_topo_graph
  						.getHalfEdgeTwin(half_edge));
  
  		if ((faceParentage & id_a) != 0 && (twinFaceParentage & id_a) != 0)
  			m_matrix[predicate] = 1;
  	}
docs: with the boundary of area B.