New new domain notation

Last week, I told you about the new domain notation. After a little peer reviewing, it appears that this notation is not as good as it seems. The inability to negate an expression (and not just a leaf) is more problematic that I have imagined. Thus, after a tiny discussion with Fabien, we decided to implement a (nearly) true polish notation for the domains. From now, the domains look more like the older ones that in the last week notation. This means there is no more extra parenthesis.
In addition of the classic leafs, you can add operators.

« Wait... what do you name leafs? »
A leaf, is a tuple of 3 elements composed of:

1. a field name
2. an operator (as string)
   
3. a value

Example:

('active', '=', True)
('foo', '!=', 'bar')
('id', 'in', [7, 13, 42, 666])
('id', '=', False)
('name', 'like', 'tiny')


« And which operators are allowed? »
Like the last week notation, the AND ('&') and OR ('|') operators are allowed . Unlike the last week notation, the arity of these operators is fixed to 2.
And yes, the one you have all been waiting for is now in: The NOT ('!') operator. This one has an arity of 1.
For compatibility with existing domains, the AND operator is applied by default.

« So, what looks like this new new notation? »
Some examples:

['&', ('active', '=', True), ('value', '!=', 'foo')]
['|', ('active', '=', True), ('state', 'in', ['open', 'draft'])
['&', ('active', '=', True), '|', '!', ('state', '=', 'closed'), ('state', '=', 'draft')]
['|', '|', ('state', '=', 'open'), ('state', '=', 'closed'), ('state', '=', 'draft')]
['!', '&', '!', ('id', 'in', [42, 666]), ('active', '=', False)]


PS: Tanks to Najlâa and Olivier for the review.