/// is this position in the paragraph right-to-left?
bool isRTL(CursorSlice const & sl, bool boundary) const;
/// is between pos-1 and pos an RTL<->LTR boundary?
/// is this position in the paragraph right-to-left?
bool isRTL(CursorSlice const & sl, bool boundary) const;
/// is between pos-1 and pos an RTL<->LTR boundary?