code: function(vertices, opt, linkView) {
  
              if (!util.isFunction(joint.routers.orthogonal)) {
                  throw new Error('Manhattan requires the orthogonal router as default fallback.');
              }
  
              return joint.routers.orthogonal(vertices, util.assign({}, config, opt), linkView);
          }
docs: a router to use when the manhattan router fails (one of the partial routes returns null)