message: Finish writing up a comment
diff: @@ -160,7 +160,8 @@ public:
  virtual PositionType findElementFlatIndex(PositionType pos1) const = 0;
  /**
  - * \brief Finds the range of indices o
  + * \brief Finds the range of indices of valid elements in the second set,
  + * given the index of an element in the first set.
  * \param Position of the element in the first set
  *
  * \return A range set of the positions in the second set
  