Return the next character from ``header`` at the given position.

    Return ``None`` at the end of ``header``.

    We never need to peek more than one character ahead.

def peek_ahead(header: str, pos: int) -> Optional[str]:
    """
    Return the next character from ``header`` at the given position.

    Return ``None`` at the end of ``header``.

    We never need to peek more than one character ahead.

    """
    return None if pos == len(header) else header[pos]