def modify(code: str, docstring: str):
    """
    Add or replace the docstring of the first class or function definition in the code.

    Parameters:
    code (str): The original Python code.
    new_docstring (str): The new docstring to be added or replaced.

    Returns:
    str: The modified Python code with the new docstring.
    """
    import re

    # Pattern to match class or function definitions
    pattern = re.compile(
        r'^(class\s+\w+|def\s+\w+)\s*\(.*?\)[^:]*:\s*(""".*?""")?(\s*$)+',
        re.DOTALL | re.MULTILINE,
    )

    # Format the new docstring
    if docstring == None:
        formatted_docstring = ""
    else:
        formatted_docstring = f'"""{docstring.strip()}\n"""\n'

    # Function to replace or add the docstring
    def _replace_match(match):
        existing_docstring = match.group(2)
        if existing_docstring:
            # Replace existing docstring
            return re.sub(
                r'""".*?"""(\s*$)+',
                formatted_docstring,
                match.group(0),
                count=1,
                flags=re.DOTALL | re.MULTILINE,
            )
        else:
            # Add new docstring
            if docstring == None:
                return match.group(0)
            else:
                return (
                    match.group(0)
                    + "\n"
                    + re.sub("^", "    ", formatted_docstring, flags=re.MULTILINE)
                )

    # Replace or add the docstring in the first class or function definition
    new_code, num_replacements = pattern.subn(_replace_match, code, count=1)

    return new_code
