Environment variables.

Sourcetype var = string
Sourcetype info
Sourceval info : ?deprecated:string -> ?docs:string -> ?doc:string -> var -> info
Sourceval info_var : info -> string
Sourceval info_doc : info -> string
Sourceval info_docs : info -> string
Sourceval info_deprecated : info -> string option
Sourcemodule Set : Stdlib.Set.S with type elt = info