message: Avoid v[0] for persistence of empty complex
  It was reported by some debug mode while I was working on something else
diff: @@ -100,7 +100,7 @@ class Persistent_cohomology {
  ds_rank_(num_simplices_), // union-find
  ds_parent_(num_simplices_), // union-find
  ds_repr_(num_simplices_, NULL), // union-find -> annotation vectors
  - dsets_(&ds_rank_[0], &ds_parent_[0]), // union-find
  + dsets_(ds_rank_.data(), ds_parent_.data()), // union-find
  cam_(), // collection of annotation vectors
  zero_cocycles_(), // union-find -> Simplex_key of creator for 0-homology
  transverse_idx_(), // key -> row
  