message: Try once more to clarify what 'keys' do in the FilteredComplex concept.
diff: @@ -29,10 +29,6 @@ struct FilteredComplex
  {
  /** Handle to specify a simplex. */
  typedef unspecified Simplex_handle;
  -/** \brief Key associated to each simplex.
  - *
  - * Must be an integer type. */
  - typedef unspecified Simplex_key;
  /** \brief Type for the value of the filtration function.
  *
  * Must be comparable with <. */
  @@ -58,20 +54,10 @@ struct FilteredComplex
  * filtration function on the complex. */
  Filtration_value filtration(Simplex_handle sh);
  -/** \brief Returns a key that is different from the keys associated
  - * to the simplices. */
  - Simplex_key null_key ();
  -/** \brief Returns the key associated to a simplex.
  - *
  - * This is never called on null_simplex(). */
  - Simplex_key key ( Simplex_handle sh );
  /** \brief Returns the simplex that has index idx in the filtration.
  *
  * This is only called on valid indices. */
  Simplex_handle simplex ( size_t idx );
  -/** \brief Assign a key to a simplex. */
  - void assign_key(Simplex_handle sh, Simplex_key key);
  -
  /** \brief Iterator on the simplices belonging to the
  * boundary of a simplex.
  *
  @@ -115,6 +101,26 @@ typedef unspecified Filtration_simplex_range;
  * .begin() and .end() return type Filtration_simplex_iterator.*/
  Filtration_simplex_range filtration_simplex_range();
  +/** \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 `num_simplices()`. Suggested value: -1. */
  + Simplex_key null_key ();
  +/** \brief Returns the number stored for a simplex by `assign_key`.
  + *
  + * This is never called on null_simplex(). */
  + Simplex_key key ( Simplex_handle sh );
  +/** \brief Store a number for a simplex, which can later be retrieved with `key(sh)`.
  + *
  + * This is never called on null_simplex(). */
  + void assign_key(Simplex_handle sh, Simplex_key n);
  +/** @} */
  +
  /* \brief Iterator over the simplices of the complex,
  * in an arbitrary order.
  