expression-oriented syntax for imp/datalog

gistfile1.txt

?an:id can fly
= ?an is a bird
- ?an is a penguin
+ ?an is Harry the Rocket Penguin
// nb. left-associative; a - b + c = (a - b) + c.

safe path from ?a to ?c 
= edge from ?a to ?c
+ (edge from ?a to ?b
   - ?b is dangerous
   & safe path from ?b to ?c)

?p:text works at alice corp but not evil eve studios
= ?p works at "alice corp" for _ USD
- ?p works at "evil eve studios" for _ USD

?p:text only works at alice corp
= ?p works at "alice corp" for _ USD
- (?p works at ?c for _ USD
   & ?c != "alice corp")