A few `DAG.__getitem__` "edge" cases.

[thorwhalen]

It seems like order get's screwed up. See below that dag[:'b c'] gives the wrong anwer (the same thatdag[:'c b]` gives).

Also, in dag[:'b d'] one would expect to get the value of b and the value of d, but we only get d..

[thorwhalen]

See also #60