_groupByMatchingIdentifiers(groups) {
    return this._group(groups, (nodes) => {
      return nodes
        .filter(node => node.name)
        .map(node => node.name)
        .join(':');
    });
  }
Iterates over the multi-dimensional array of nodes, and returns a new
array grouping them based on matching identifiers.

@private

@param   {Node[][][]} groups
@returns {Node[][][]}