getCommonRings(vertexA, vertexB) {
    let commonRings = Array();

    for (var i = 0; i < vertexA.value.rings.length; i++) {
      for (var j = 0; j < vertexB.value.rings.length; j++) {
        if (vertexA.value.rings[i] == vertexB.value.rings[j]) {
          commonRings.push(vertexA.value.rings[i]);
        }
      }
    }

    return commonRings;
  }
Returns an array of ring ids shared by both vertices.

@param {Vertex} vertexA A vertex.
@param {Vertex} vertexB A vertex.
@returns {Number[]} An array of ids of rings shared by the two vertices.