What is syntax error in line 'elif item is not None:'

edit

Example:

def _flatten(seq):
    """Internal function."""
    res = ()
    for item in seq:
        if is instance(item, (tuple, list)):
            res = res + _flatten(item)
elif item is not None:
        res = res + (item,)
    return res

try: _flatten = _tkinter._flatten
except AttributeError: pass