diff --git a/README.md b/README.md
index 5eaa63b..f38c512 100644
--- a/README.md
+++ b/README.md
@@ -27,7 +27,7 @@ dependency:
   <groupId>org.microbean</groupId>
   <artifactId>microbean-assign</artifactId>
   <!-- Always check https://search.maven.org/artifact/org.microbean/microbean-bean for up-to-date available versions. -->
-  <version>0.0.1</version>
+  <version>0.0.3</version>
 </dependency>
 ```
 
diff --git a/src/main/java/org/microbean/assign/SupertypeList.java b/src/main/java/org/microbean/assign/SupertypeList.java
index c503c6e..4405ec5 100644
--- a/src/main/java/org/microbean/assign/SupertypeList.java
+++ b/src/main/java/org/microbean/assign/SupertypeList.java
@@ -28,16 +28,23 @@
  */
 public final class SupertypeList extends AbstractList<TypeMirror> {
 
+  private final int interfaceIndex;
+  
   private final List<TypeMirror> sortedSupertypes;
 
-  SupertypeList(final List<? extends TypeMirror> sortedSupertypes) {
+  SupertypeList(final List<? extends TypeMirror> sortedSupertypes, final int interfaceIndex) {
     super();
     if (sortedSupertypes.isEmpty()) {
       throw new IllegalArgumentException();
     }
     switch (sortedSupertypes) {
-    case SupertypeList sl -> this.sortedSupertypes = sl.sortedSupertypes;
-    default -> this.sortedSupertypes = List.copyOf(sortedSupertypes);
+    case SupertypeList sl:
+      this.sortedSupertypes = sl.sortedSupertypes;
+      this.interfaceIndex = sl.interfaceIndex;
+      break;
+    default:
+      this.sortedSupertypes = List.copyOf(sortedSupertypes);
+      this.interfaceIndex = interfaceIndex;
     }    
   }
 
@@ -46,6 +53,17 @@ public final TypeMirror get(final int index) {
     return this.sortedSupertypes.get(index);
   }
 
+  /**
+   * Returns the index of the first {@linkplain javax.lang.model.element.ElementKind#isInterface() interface type} this
+   * {@link SupertypeList} contains, or a negative value if it contains no interface types.
+   *
+   * @return the index of the first {@linkplain javax.lang.model.element.ElementKind#isInterface() interface type} this
+   * {@link SupertypeList} contains, or a negative value if it contains no interface types
+   */
+  public final int interfaceIndex() {
+    return this.interfaceIndex;
+  }
+  
   @Override // AbstractList<TypeMirror>
   public final int size() {
     return this.sortedSupertypes.size();
diff --git a/src/main/java/org/microbean/assign/Types.java b/src/main/java/org/microbean/assign/Types.java
index d86b53e..8d32f95 100644
--- a/src/main/java/org/microbean/assign/Types.java
+++ b/src/main/java/org/microbean/assign/Types.java
@@ -277,10 +277,12 @@ public final SupertypeList supertypes(final TypeMirror t, final Predicate<? supe
     supertypes(t, p, nonInterfaceTypes, interfaceTypes, newHashSet(13)); // arbitrary size
     nonInterfaceTypes.trimToSize();
     interfaceTypes.trimToSize();
+    final int interfaceIndex = interfaceTypes.isEmpty() ? -1 : nonInterfaceTypes.size();
     return
       new SupertypeList(concat(nonInterfaceTypes.stream(), // non-interface supertypes are already sorted from most-specific to least
                                interfaceTypes.stream().sorted(this.c)) // have to sort interfaces because you can extend them in any order
-                        .toList());
+                        .toList(),
+                        interfaceIndex);
   }
 
   private final void supertypes(final TypeMirror t,