def isplit(string, sep='\t\n\x0b\x0c\r '):
    """Returns an iterator over string.split().

    This is efficient in combination with cooccurrence(), since the
    string may be very long (e.g., Brown corpus).

    """
    a = []
    for ch in string:
        if ch not in sep:
            a.append(ch)
            continue
        if a:
            yield ''.join(a)
            a = []
    if a:
        yield ''.join(a)
