Allow rules to have multiple conclusions

[robsimmons]

It's pretty clear what we want to have happen here:

H1, H2, H3 :- P1, P2, P3. 

should have the same meaning as

H1 :- P1, P2, P3. 
H2 :- P1, P2, P3. 
H3 :- P1, P2, P3. 

This can be treated as syntactic sugar and would almost certainly be compiled away by some compiler phase, but would be nice sugar to have.