Return the hash for the ancestor block with number ``block_number``.
        Return the empty bytestring ``b''`` if the block number is outside of the
        range of available block numbers (typically the last 255 blocks).

def get_ancestor_hash(self, block_number: int) -> Hash32:
        """
        Return the hash for the ancestor block with number ``block_number``.
        Return the empty bytestring ``b''`` if the block number is outside of the
        range of available block numbers (typically the last 255 blocks).
        """
        ancestor_depth = self.block_number - block_number - 1
        is_ancestor_depth_out_of_range = (
            ancestor_depth >= MAX_PREV_HEADER_DEPTH or
            ancestor_depth < 0 or
            block_number < 0
        )
        if is_ancestor_depth_out_of_range:
            return Hash32(b'')

        try:
            return nth(ancestor_depth, self.execution_context.prev_hashes)
        except StopIteration:
            # Ancestor with specified depth not present
            return Hash32(b'')