Rule: constant conditions

[anderseknert]

allow {
    1 == 1
    true
    false
}

And so on...

Note that we can't do much about a single true in an otherwise empty body, as:

allow := true

will be rewritten as:

allow = true {
    true
}

So we can accept that as an exception for now.

[anderseknert]

This is mostly fixed with #96 , but we only do "top level" checks in rules with that, so I'm leaving this open hoping to come back to it later for checks in nestee expressions.