def replace_non_ascii_seqs(word: str, placeholder: str) ->str:
    """
    >>> replace_non_ascii_seqs("","÷")
    ''

    >>> replace_non_ascii_seqs("Ü", "÷")
    '÷'

    >>> replace_non_ascii_seqs("Üüø", "÷")
    '÷'

    >>> replace_non_ascii_seqs("abcd", "÷")
    'abcd'

    >>> replace_non_ascii_seqs("aæbñńcdú", "÷")
    'a÷b÷cd÷'

    >>> replace_non_ascii_seqs("any_string", "÷ ")
    Traceback (most recent call last):
    ...
    ValueError: Placeholder should be a single character, but is ÷ 
    """
    if len(placeholder) != 1:
        raise ValueError(
            f'Placeholder should be a single character, but is {placeholder}')
    new_word = ''
    ongoing_non_ascii_seq = False
    for ch in word:
        if ord(ch) < 128:
            if ongoing_non_ascii_seq:
                new_word += placeholder
                ongoing_non_ascii_seq = False
            new_word += ch
        else:
            ongoing_non_ascii_seq = True
    if ongoing_non_ascii_seq:
        new_word += placeholder
    return new_word
