code: @NonNull
    public final <X> GenericType<T> where(
        @NonNull GenericTypeParameter<X> freeVariable, @NonNull Class<X> actualType) {
      return where(freeVariable, GenericType.of(actualType));
    }
docs: Substitutes a free type variable with an actual type. See {@link GenericType this class's
  javadoc} for an example.