+ /// move to next position
+ void forwardPos();
+ /// move to previous position
+ void backwardPos();
+ /// move to next cell
+ void forwardIdx();
+ /// move to previous cell
+ void backwardIdx();
+ /// are we at the end of the cell
+ bool at_cell_end() const;
+ /// are we at the start of the cell
+ bool at_cell_begin() const;
+ /// are we at the end of this slice
+ bool at_end() const;
+ /// are we at the start of this slice
+ bool at_begin() const;
+