This module is used to implement Context_free.V1.map_top_down

`type 'a t`````
`val convert : 'a t Ppxlib__.Import.list ‑> loc:Location.t ‑> path:Ppxlib__.Import.string ‑> Ppxlib__.Import.extension ‑> 'a Ppxlib__.Import.option`
`val convert_inline : 'a t Ppxlib__.Import.list ‑> loc:Location.t ‑> path:Ppxlib__.Import.string ‑> Ppxlib__.Import.extension ‑> 'a Ppxlib__.Import.list Ppxlib__.Import.option`