# File lib/ai4r/clusterers/single_linkage.rb, line 146
      def merge_clusters(index_a, index_b, index_clusters)
        index_a, index_b = index_b, index_a if index_b > index_a
        new_index_cluster = index_clusters[index_a] +
          index_clusters[index_b]
        index_clusters.delete_at index_a
        index_clusters.delete_at index_b
        index_clusters << new_index_cluster
        return index_clusters
      end