The selection number. The selected regions are assigned numbers that correspond to how far the region is from the start of the text. The selected region closest to the beginning of the text region is assigned the number 0, etc. Note that adding, moving or deleting a selected region can change the numbering.
the new start position of the selection
the new end position of (e.g. offset immediately past) the selection
Return: %TRUE if success, %FALSE otherwise
Changes the start and end offset of the specified selection.