Safety check incorrect for set comprehensions

[tsandall]

It seems the safety check for set comprehensions is not working as expected.

> {k | a =[1,2,3]; a[k] }
2 errors occurred:
1:2: rego_unsafe_var_error: var k is unsafe
1:2: rego_unsafe_var_error: var a is unsafe

Whereas the following is OK:

> a=[1,2,3]; x = {k | a[k]}
+---------+---------+
|    a    |    x    |
+---------+---------+
| [1,2,3] | [0,1,2] |
+---------+---------+