def METHOD_NAME(
    expected_difficulty: int,
    header: BlockHeaderAPI,
    signer: Address,
    signers: Sequence[Address],
) -> str:
    return (
        f"Expected difficulty of {header} to be {expected_difficulty} "
        f"but was {header.difficulty}.\n"
        f"Header signer: {encode_hex(signer)}. Valid signers: {signers}"
    )