diff --git a/reusablecfg/src/main/jastadd/IdentityTupleSet.jadd b/reusablecfg/src/main/jastadd/IdentityTupleSet.jadd
new file mode 100644
index 0000000000000000000000000000000000000000..02ed2d12d4bce7a165cb387b6ca4aa898ac7613e
--- /dev/null
+++ b/reusablecfg/src/main/jastadd/IdentityTupleSet.jadd
@@ -0,0 +1,102 @@
+aspect IdentityTupleSet {
+
+  /**
+   * Set that contains either one or two unique objects. The objects are
+   * compared with reference equality.
+   */
+  class IdentityTupleSet<E> implements Set<E> {
+    final E a, b;
+
+    public IdentityTupleSet(E a, E b) {
+      this.a = a;
+      this.b = b;
+    }
+
+    @Override
+    public boolean add(E e) {
+      throw new UnsupportedOperationException();
+    }
+    @Override
+    public boolean addAll(Collection<? extends E> e) {
+      throw new UnsupportedOperationException();
+    }
+    @Override
+    public void clear() {
+      throw new UnsupportedOperationException();
+    }
+    @Override
+    public boolean contains(Object o) {
+      return o == a || o == b;
+    }
+    @Override
+    public boolean containsAll(Collection<?> c) {
+      for (Object o : c) {
+        if (!contains(o)) {
+          return false;
+        }
+      }
+      return true;
+    }
+    @Override
+    public int hashCode() {
+      return a.hashCode() + b.hashCode();
+    }
+    @Override
+    public boolean isEmpty() {
+      return false;
+    }
+    @Override
+    public Iterator<E> iterator() {
+      return new Iterator<E>() {
+        int index = 0;
+        @Override
+        public boolean hasNext() {
+          return (a == b && index < 1)
+              || (a != b && index < 2);
+        }
+        @Override
+        public E next() {
+          return ++index == 1 ? a : b;
+        }
+        @Override
+        public void remove() {
+          throw new UnsupportedOperationException();
+        }
+      };
+    }
+    @Override
+    public boolean remove(Object o) {
+      throw new UnsupportedOperationException();
+    }
+    @Override
+    public boolean removeAll(Collection<?> c) {
+      throw new UnsupportedOperationException();
+    }
+    @Override
+    public boolean retainAll(Collection<?> c) {
+      throw new UnsupportedOperationException();
+    }
+    @Override
+    public int size() {
+      return a == b ? 1 : 2;
+    }
+    @Override
+    public Object[] toArray() {
+      return new Object[] { a, b };
+    }
+    @Override
+    public <T> T[] toArray(T[] array) {
+      array[0] = (T) a;
+      array[1] = (T) b;
+      return array;
+    }
+    @Override
+    public String toString() {
+      if (a == b) {
+        return "[" + a + "]";
+      } else {
+        return "[" + a + ", " + b + "]";
+      }
+    }
+  }
+}