Move current position backwards by length.

def spew_length(self, length):
        """Move current position backwards by length."""
        pos = self.pos
        if not pos or length > pos:
            return None

        row = self.row
        for char in reversed(self.string[pos - length:pos]):
            pos -= 1
            if char == '\n':  # handle a newline char
                row -= 1

        self.pos = pos
        self.col = self.eol_distance_last()
        self.row = row

        if self.has_space():  # Set eos if there is no more space left.
            self.eos = 0