new Function<Optional<?>, @Nullable Object>() {
        @Override
        public @Nullable Object apply(Optional<?> optional) {
          return optional.orNull();
        }
      };

  /**
   * Sets.cartesianProduct doesn't allow sets that contain null, but we want null to mean "don't

constraints.Future.message = {item} gelecek bir değer olmalıdır.
constraints.Max.message = {item}, {value} değerine eşit veya küçük olmalıdır.
constraints.Min.message = {item}, {value} değerine eşit veya büyük olmalıdır.
constraints.NotNull.message = {item} gereklidir.
constraints.Null.message = {item} boş olmalıdır.
constraints.Past.message = {item} geçmiş bir değer olmalıdır.
constraints.Pattern.message = {item}, "{regexp}" ile eşleşmiyor.

constraints.Future.message = {item} deve essere una data futura.
constraints.Max.message = {item} deve essere minore o uguale a {value}.
constraints.Min.message = {item} deve essere maggiore o uguale a {value}.
constraints.NotNull.message = {item} è obbligatorio.
constraints.Null.message = {item} deve essere nullo.
constraints.Past.message = {item} deve essere una data passata.
constraints.Pattern.message = {item} non corrisponde a "{regexp}".

constraints.Future.message = {item} musi być wartością przyszłą.
constraints.Max.message = {item} musi być mniejsze lub równe {value}.
constraints.Min.message = {item} musi być większe lub równe {value}.
constraints.NotNull.message = {item} jest niewypełnione.
constraints.Null.message = {item} musi być null.
constraints.Past.message = {item} musi być wartością przeszłą.
constraints.Pattern.message = {item} nie pasuje do "{regexp}".

constraints.Future.message = {item} doit être une valeur future.
constraints.Max.message = {item} doit être inférieur ou égal à {value}.
constraints.Min.message = {item} doit être supérieur ou égal à {value}.
constraints.NotNull.message = {item} est requis.
constraints.Null.message = {item} doit être nul.
constraints.Past.message = {item} doit être une valeur passée.
constraints.Pattern.message = {item} ne correspond pas à « {regexp} ».

constraints.Digits.message = {item} 必须是数字。(预期: <数字>.<数字>)
constraints.Future.message = {item} 必须是未来的值。
constraints.Max.message = {item} 必须小于或等于 {value}。
constraints.Min.message = {item} 必须大于或等于 {value}。
constraints.NotNull.message = {item} 不能为空。
constraints.Null.message = {item} 必须为 null。
constraints.Past.message = {item} 必须是过去的值。
constraints.Pattern.message = {item} 不匹配 "{regexp}"。
constraints.Size.message = {item} 的大小必须在 {min} 到 {max} 个字符之间。

constraints.Future.message = {item} harus berupa nilai masa depan.
constraints.Max.message = {item} harus kurang dari atau sama dengan {value}.
constraints.Min.message = {item} harus lebih dari atau sama dengan {value}.
constraints.NotNull.message = {item} wajib diisi.
constraints.Null.message = {item} harus null.
constraints.Past.message = {item} harus berupa nilai masa lalu.
constraints.Pattern.message = {item} tidak cocok dengan "{regexp}".

constraints.Future.message = {item} muss ein zukünftiger Wert sein.
constraints.Max.message = {item} muss kleiner oder gleich {value} sein.
constraints.Min.message = {item} muss größer oder gleich {value} sein.
constraints.NotNull.message = {item} ist erforderlich.
constraints.Null.message = {item} muss null sein.
constraints.Past.message = {item} muss ein vergangener Wert sein.
constraints.Pattern.message = {item} stimmt nicht mit "{regexp}" überein.

constraints.Future.message = {item} must be a future value.
constraints.Max.message = {item} must be less than or equal to {value}.
constraints.Min.message = {item} must be greater than or equal to {value}.
constraints.NotNull.message = {item} is required.
constraints.Null.message = {item} must be null.
constraints.Past.message = {item} must be a past value.
constraints.Pattern.message = {item} does not match "{regexp}".

constraints.Future.message = {item} must be a future value.
constraints.Max.message = {item} must be less than or equal to {value}.
constraints.Min.message = {item} must be greater than or equal to {value}.
constraints.NotNull.message = {item} is required.
constraints.Null.message = {item} must be null.
constraints.Past.message = {item} must be a past value.
constraints.Pattern.message = {item} does not match "{regexp}".