private DeclaredType toDeclaredType(TypeMirror typeMirror) {
    verify(typeMirror.getKind() == TypeKind.DECLARED || typeMirror.getKind() == TypeKind.ERROR);
    return (DeclaredType) typeMirror;
  }
overridable stringify.
@param input the input
@param context the context
@return the string
generator/src/org/immutables/generator/TypeHierarchyCollector.java