code: @NotNull
      public OptionalLong filter(@NotNull LongPredicate predicate) {
          if (!isPresent()) return this;
          return predicate.test(value) ? this : OptionalLong.empty();
      }
docs: Performs filtering on inner value if it is present.
  
  @param predicate  a predicate function
  @return this {@code OptionalLong} if the value is present and matches predicate,
  otherwise an empty {@code OptionalLong}