Bug: type-constraint not applied

**Expected and real behaviour**: 

query `as_boolean(prime, x) = @false` must produce only natural solutions for `x` due to prime signature `?prime(natural)`.

but it also produces integer results.

**How to reproduce**: 

```
as_boolean(prime, x) = @false
-- to fix 'x' can be predefined
x ~ natural, as_boolean(prime, x) = @false
```

## System info

laurelang: dev

OS: unix
