message: Simplex_key clarifications in FilteredComplex concept.
diff: @@ -31,7 +31,7 @@ struct FilteredComplex
  typedef unspecified Simplex_handle;
  /** \brief Key associated to each simplex.
  *
  - * Must be a signed integer type. */
  + * Must be an integer type. */
  typedef unspecified Simplex_key;
  /** \brief Type for the value of the filtration function.
  *
  @@ -67,8 +67,8 @@ struct FilteredComplex
  Simplex_key key ( Simplex_handle sh );
  /** \brief Returns the simplex that has index idx in the filtration.
  *
  - * This is never called on null_key(). */
  - Simplex_handle simplex ( Simplex_key idx );
  + * 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);
  