getRingbondType(vertexA, vertexB) {
    // Checks whether the two vertices are the ones connecting the ring
    // and what the bond type should be.
    if (vertexA.value.getRingbondCount() < 1 || vertexB.value.getRingbondCount() < 1) {
      return null;
    }

    for (var i = 0; i < vertexA.value.ringbonds.length; i++) {
      for (var j = 0; j < vertexB.value.ringbonds.length; j++) {
        // if(i != j) continue;
        if (vertexA.value.ringbonds[i].id === vertexB.value.ringbonds[j].id) {
          // If the bonds are equal, it doesn't matter which bond is returned.
          // if they are not equal, return the one that is not the default ("-")
          if (vertexA.value.ringbonds[i].bondType === '-') {
            return vertexB.value.ringbonds[j].bond;
          } else {
            return vertexA.value.ringbonds[i].bond;
          }
        }
      }
    }

    return null;
  }
Returns the type of the ringbond (e.g. '=' for a double bond). The ringbond represents the break in a ring introduced when creating the MST. If the two vertices supplied as arguments are not part of a common ringbond, the method returns null.

@param {Vertex} vertexA A vertex.
@param {Vertex} vertexB A vertex.
@returns {(String|null)} Returns the ringbond type or null, if the two supplied vertices are not connected by a ringbond.