Fix error message on failed unification

Sometimes, unification fails with a nonsensical error message like this.

    %***************************** failure **************************
    %**
    %** Tell: [1] = [1]

It appears that rebinding is not properly undone, and the two values are
still aliased when producing the error message. Hence the confusing
output.

XXX: This fix works but I have no idea why.

platform-test/CMakeLists.txt

@@ -13,6 +13,7 @@ set(BASE_FUNCTORS
     #"finalize.oz" "gc.oz"
     "state.oz" "thread.oz"
     #"vm.oz" #FIXME vm tests are buggy, see #313.
+    "unify.oz"
     "reflection.oz" "serializer.oz"
 )
 file(MAKE_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/base")

platform-test/base/unify.oz

@@ -0,0 +1,28 @@
+functor
+import
+   VM
+export
+   Return
+define
+   Return = unify([vmlist(proc {$}
+			     try
+				{VM.list} = unit
+				fail
+			     catch failure(debug:d(info:[eq(unit _)] ...) ...) then
+				skip
+			     end
+			  end
+			  keys:[unify])
+                   order(proc {$}
+			    fun {Const X} [1 2 X] end
+			 in
+			    try
+			       {Const 1} = {Const 2}
+			       fail
+			    catch failure(debug:d(info:[eq(1 2)] ...) ...) then
+			       skip
+			    end
+			 end
+			 keys:[unify])
+		  ])
+end

vm/vm/main/unify.cc

@@ -465,8 +465,16 @@ bool StructuralDualWalk::processPair(VM vm, RichNode left, RichNode right) {
 }
 
 void StructuralDualWalk::rebind(VM vm, RichNode left, RichNode right) {
-  rebindTrail.push_back(vm, left.makeBackup());
-  left.reinit(vm, right);
+  // XXX: The test is to work around `a.reinit(vm, b)` where `b` is sometimes
+  // modified. It is better to reinit Unstable nodes than Stable ones anyway.
+  // See #312 for details.
+  if (right.isStable()) {
+      rebindTrail.push_back(vm, left.makeBackup());
+      left.reinit(vm, right);
+  } else {
+      rebindTrail.push_back(vm, right.makeBackup());
+      right.reinit(vm, left);
+  }
 }
 
 void StructuralDualWalk::cleanupOnFailure(VM vm) {