private void collectUnresolvedInterface(TypeMirror typeMirror, TypevarContext context) {
    if (typeMirror.getKind() == TypeKind.ERROR) {
      DeclaredType declaredType = toDeclaredType(typeMirror);
      String stringified = stringify(declaredType, context);
      implementedInterfaceNames.add(stringified);
    }
  }
overridable stringify.
@param input the input
@param context the context
@return the string
generator/src/org/immutables/generator/TypeHierarchyCollector.java