EQ conditions which match against a negative result fail to find any results.

Description

I was trying to follow the example provided by [this blog post](https://mattsenior.com/2014/02/using-clojures-core-logic-to-solve-simple-number-puzzles) and found that it only worked if I removed the conditions that test for subtraction with a negative result. I removed a lot of different things and found that even this basic example does not find any matches:

```
(ns example.core
(:refer-clojure :exclude [==])
(:require [clojure.core.logic :refer :all])
(:require [clojure.core.logic.fd :as fd]))

(run* [q]
(fresh [a0 a1]
(== q [a0 a1])
(fd/in a0 a1 (fd/interval 1 9))
(fd/- a0 a1 -1)
;(fd/eq
; (= (- a0 a1) -1))
))
```

Dependencies (though I tried clojure 1.8 and 1.9 too):
```
:dependencies [[org.clojure/clojure "1.7.0"]
[org.clojure/core.logic "0.8.11"]]
```

Environment

OS: Mac OSX
JVM:
java -version
java version "1.8.0_91"
Java(TM) SE Runtime Environment (build 1.8.0_91-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.91-b14, mixed mode)

Status

Assignee

David Nolen

Reporter

Stephen Hopper

Labels

Approval

None

Patch

None

Priority

Major
Configure