Despite lots of arguments to the contrary,
6 and 1 in lis means
6 and (1 in lis) It does not mean:
(6 and 1) in lis The page that Maroun Maroun linked to in his comments indicates that and has a lower precedence than in, so the in will be evaluated first.
You can test it like this:
0 and 1 in [0] If this means (0 and 1) in [0] then it will evaluate to true, because zero is in [0].
If it means 0 and (1 in [0]) then it will evaluate to zero, because 0 is false.
It evaulates to zero.