+ /// returns the text to be used as tooltip
+ /// \param prefix: a string that will preced the tooltip,
+ /// e.g., "Index: ".
+ /// \param numlines: the number of lines in the tooltip
+ /// \param len: length of those lines
+ docstring toolTipText(docstring prefix = empty_docstring(),
+ size_t numlines = 5, size_t len = 80) const;