Jsont_typegist.JsontCustom Jsont.t value metadata key.
If a gist or field defines a value for this key, the Jsont_typegist.to_jsont function uses it as the derived Jsont.t value.
Using a JSON type that always (de)serizalizes null maps with a constant value like Jsont.ignore does can also be used to indicate that a value should not be serialized.