def chainwork_of_header_at_height(self, height: int) -> int:
        """work done by single header at given height"""
        chunk_idx = height // CHUNK_SIZE - 1
        target = self.get_target(chunk_idx)
        work = ((2 ** 256 - target - 1) // (target + 1)) + 1
        return work
work done by single header at given height
electrum/blockchain.py