isTerminal() {
    if (this.value.hasAttachedPseudoElements) {
      return true;
    }

    return this.parentVertexId === null && this.children.length < 2 || this.children.length === 0;
  }
Returns true if this vertex is terminal (has no parent or child vertices), otherwise returns false. Always returns true if associated value has property hasAttachedPseudoElements set to true.

@returns {Boolean} A boolean indicating whether or not this vertex is terminal.