Generic.UnmagicCustom unmagic key.
The Fun.Generic.unmagic function consults this key in the metadata of a gist g before unmagicing the value.
val mem : ('a, 'b) Type.Gist.Meta.t2 -> boolval add : 'b t -> ('a, 'b) Type.Gist.Meta.t2 -> ('a, 'b) Type.Gist.Meta.t2val find : ('a, 'b) Type.Gist.Meta.t2 -> 'b t optionval remove : ('a, 'b) Type.Gist.Meta.t2 -> ('a, 'b) Type.Gist.Meta.t2