'Move the cursor down one line in the same column. If the
cursor is at the last line, create a new line at the bottom.'

def index(self):

    (top, bottom) = self.margins
    if (self.cursor.y == bottom):
        self.buffer.pop(top)
        self.buffer.insert(bottom, take(self.columns, self.default_line))
    else:
        self.cursor_down()
