static <A> Collection<A> treeSetClusters_add(Map<A, Collection<A>> clusters, L<A> l) {
  for (int i = 0; i < l(l)-1; i++)
    treeSetClusters_add(clusters, l.get(i), l.get(i+1));
  ret clusters.get(first(l));
}

static <A> void treeSetClusters_add(Map<A, Collection<A>> clusters, A a, A b) {
  if (eq(a, b)) {
    Collection<A> cluster = clusters.get(a);
    if (cluster == null)
      clusters.put(a, littreeset(a));
    ret;
  }
  
  Collection<A> clusterA = clusters.get(a);
  Collection<A> clusterB = clusters.get(b);
  if (clusterA == null && clusterB == null) {
    L<A> cluster = ll(a, b);
    clusters.put(a, cluster);
    clusters.put(b, cluster);
  } else if (clusterA == null) {
    clusterB.add(a);
    clusters.put(a, clusterB);
  } else if (clusterB == null) {
    clusterA.add(b);
    clusters.put(b, clusterA);
  } else if (clusterA != clusterB)
    treeSetClusters_merge(clusters, clusterA, clusterB);
}

Began life as a copy of #1009481