def fast_replace(t, sep, sub=None):
    """
    Replace separators (sep) with substitute char, sub. Many-to-one substitute.

    "a.b, c" SEP='.,'
    :param t:  input text
    :param sep: string of chars to replace
    :param sub: replacement char
    :return:  text with separators replaced
    """
    result = []
    for ch in t:
        if ch in sep:
            if sub:
                result.append(sub)
        else:
            result.append(ch)
    return ''.join(result)