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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def rol(self, bits, start=None, end=None):
 """Rotate bits to the left in-place. bits -- The number of bits to rotate by. start -- Start of slice to rotate. Defaults to 0. end -- End of slice to rotate. Defaults to self.len. Raises ValueError if bits < 0. """

        if not self.len:
            raise Error("Cannot rotate an empty bitstring.")
        if bits < 0:
            raise ValueError("Cannot rotate left by negative amount.")
        start, end = self._validate_slice(start, end)
        bits %= (end - start)
        if not bits:
            return
        lhs = self._slice(start, start + bits)
        self._delete(bits, start)
        self._insert(lhs, end - bits)