Comprehension terms not being evaluated

[tsandall]

topdown is not evaluating comprehension terms. As a result, references to vars in the comprehension escape the child context are evaluated in the outer query (resulting in errors, see below).

This can be solved relatively easily by rewriting comprehensions similar to rule heads.

$ opa run
OPA 0.5.8-dev (commit 8019c7e6, built at 2017-09-13T00:55:24Z)

Run 'help' to see a list of commands.

> vs = [[0], [1], [2]]
> [v[0] | v  = vs[_]]
storage_not_found_error: /0: document does not exist