diff --git a/src/env.ml b/src/env.ml
index cf533782..84173b93 100644
--- a/src/env.ml
+++ b/src/env.ml
@@ -80,3 +80,6 @@ let extend t ~vars =
         | Some _ -> v2
         | None -> v1)
     )
+
+let add t ~var ~value =
+  make ~base:t.base ~extra:(Map.add t.extra var value)
diff --git a/src/env.mli b/src/env.mli
index 0bb9ef9a..02c67ec6 100644
--- a/src/env.mli
+++ b/src/env.mli
@@ -16,3 +16,5 @@ val to_unix : t -> string array
 val get : t -> Var.t -> string option
 
 val extend : t -> vars:string Map.t -> t
+
+val add : t -> var:Var.t -> value:string -> t