code: public static TaggedValue newTaggedValue(Tag tag, Object value) {
          if (tag == null) {
              throw new IllegalArgumentException("tag must not be null");
          }
          return new TaggedValue(tag, value);
      }
docs: Return a tagged value for the given tag and value (some edn data).
  The tag must not be null.
  @param tag not null.
  @param value may be null.
  @return a TaggedValue, never null.