[feature-request] `:not()` to support generic selectors (not only "simple" ones)

The document (version 0.9.1) says:

:not() accepts a sequence of simple selectors, not just single simple selector. For example, :not(a.important[rel]) is allowed, even though the negation contains 3 simple selectors.

May I ask what is a simple selector? Can :not() support something like :not(a>b)?

>>> import cssselect
>>> cssselect.parse('a:not(p>a)')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.4/site-packages/cssselect/parser.py", line 355, in parse
    return list(parse_selector_group(stream))
  File "/usr/lib/python3.4/site-packages/cssselect/parser.py", line 370, in parse_selector_group
    yield Selector(*parse_selector(stream))
  File "/usr/lib/python3.4/site-packages/cssselect/parser.py", line 378, in parse_selector
    result, pseudo_element = parse_simple_selector(stream)
  File "/usr/lib/python3.4/site-packages/cssselect/parser.py", line 471, in parse_simple_selector
    raise SelectorSyntaxError("Expected ')', got %s" % (next,))
cssselect.parser.SelectorSyntaxError: Expected ')', got <DELIM '>' at 7>

Reproduce the failure with cssselect.parse('a:not(p>a)'). Start in cssselect/parser.py at parse, parse_selector_group, and parse_simple_selector, which appear in the traceback. Done means the requested generic selector form is accepted by :not() without the shown syntax error.