Add a test to assert that equals (x, x) is always true

In particular, it should always be true for arbitrarily nested
structures, where some layers have Setoid and some do not.

test/index.js

@@ -567,6 +567,33 @@ test ('Contravariant', () => {
 });
 
 test ('equals', () => {
+  const {nestedSetoidArb} = jsc.letrec (tie => ({
+    builtinSetoidArb: jsc.record ({
+      value: tie ('nestedSetoidArb'),
+    }),
+    noSetoidArb: jsc.record ({
+      'value': tie ('nestedSetoidArb'),
+      '@@type': jsc.constant ('sanctuary-type-classes/NoSetoid@1'),
+      '@@show': jsc.constant (function() { return `NoSetoid (${show (this.value)})`; }),
+    }),
+    customSetoidArb: jsc.record ({
+      'value': tie ('nestedSetoidArb'),
+      '@@type': jsc.constant ('sanctuary-type-classes/CustomSetoid@1'),
+      '@@show': jsc.constant (function() { return `CustomSetoid (${show (this.value)})`; }),
+      'fantasy-land/equals': jsc.constant (function(other) {
+        return Z.equals (this.value, other.value);
+      }),
+    }),
+    nestedSetoidArb: jsc.oneof ([
+      tie ('builtinSetoidArb'),
+      tie ('noSetoidArb'),
+      tie ('customSetoidArb'),
+      jsc.nat,
+    ]),
+  }));
+
+  nestedSetoidArb.show = show;
+
   eq (Z.equals.length, 2);
 
   eq (Z.equals (null, null), true);
@@ -686,6 +713,8 @@ test ('equals', () => {
   eq (Z.equals ($0, $0), true);
   eq (Z.equals ($0, $1), false);
   eq (Z.equals ($1, $0), false);
+
+  jsc.assert (jsc.forall (nestedSetoidArb, x => Z.equals (x, x)));
 });
 
 test ('lt', () => {