Yield items from any nested iterable; see REF.

def flatten(items,enter=lambda x:isinstance(x, list)):
    # http://stackoverflow.com/a/40857703
    # https://github.com/ctmakro/canton/blob/master/canton/misc.py
    """Yield items from any nested iterable; see REF."""
    for x in items:
        if enter(x):
            yield from flatten(x)
        else:
            yield x