'Unselect read-only parts in shell, like prompt'

def truncate_selection(self, position_from):

    position_from = self.get_position(position_from)
    cursor = self.textCursor()
    (start, end) = (cursor.selectionStart(), cursor.selectionEnd())
    if (start < end):
        start = max([position_from, start])
    else:
        end = max([position_from, end])
    self.set_selection(start, end)
