message: Update the concept of the simplicial complex
  We use the key now.
  It wouldn't be hard to use an unordered_map, but since we usually have
  an unused field key...
diff: @@ -72,6 +72,24 @@ struct SimplicialComplexForAlpha {
  /** \brief Return type of an insertion of a simplex
  */
  typedef unspecified Insertion_result_type;
  +
  + /** \name Map interface
  + * Conceptually a `std::unordered_map<Simplex_handle,std::size_t>`.
  + * @{ */
  + /** \brief Data stored for each simplex.
  + *
  + * Must be an integer type. */
  + typedef unspecified Simplex_key;
  + /** \brief Returns a constant dummy number that is either negative,
  + * or at least as large as the number of simplices. Suggested value: -1. */
  + Simplex_key null_key ();
  + /** \brief Returns the number stored for a simplex by `assign_key()`.
  + *
  + * If `assign_key()` has not been called, it must return `null_key()`. */
  + Simplex_key key ( Simplex_handle sh );
  + /** \brief Store a number for a simplex, which can later be retrieved with `key()`. */
  + void assign_key(Simplex_handle sh, Simplex_key n);
  + /** @} */
  };
  } // namespace alpha_complex
  