diff --git a/src/install.ml b/src/install.ml
index 35490568..494750ba 100644
--- a/src/install.ml
+++ b/src/install.ml
@@ -47,6 +47,35 @@ module Section = struct
       ; "man"        , Man
       ; "misc"       , Misc
       ]
+
+  module Paths = struct
+    let lib         = Path.(relative root) "lib"
+    let libexec     = Path.(relative root) "lib"
+    let bin         = Path.(relative root) "bin"
+    let sbin        = Path.(relative root) "sbin"
+    let toplevel    = Path.(relative root) "lib/toplevel"
+    let share       = Path.(relative root) "share"
+    let share_root  = Path.(relative root) "share_root"
+    let etc         = Path.(relative root) "etc"
+    let doc         = Path.(relative root) "doc"
+    let stublibs    = Path.(relative root) "lib/stublibs"
+    let man         = Path.(relative root) "man"
+  end
+
+  let install_dir t ~package =
+    match t with
+    | Bin        -> Paths.bin
+    | Sbin       -> Paths.sbin
+    | Toplevel   -> Paths.toplevel
+    | Share_root -> Paths.share_root
+    | Stublibs   -> Paths.stublibs
+    | Man        -> Paths.man
+    | Lib        -> Path.relative Paths.lib     package
+    | Libexec    -> Path.relative Paths.libexec package
+    | Share      -> Path.relative Paths.share   package
+    | Etc        -> Path.relative Paths.etc     package
+    | Doc        -> Path.relative Paths.doc     package
+    | Misc       -> invalid_arg "Install.Section.install_dir"
 end
 
 module Entry = struct
@@ -82,36 +111,8 @@ module Entry = struct
 
   let set_src t src = { t with src }
 
-  module Paths = struct
-    let lib         = Path.(relative root) "lib"
-    let libexec     = Path.(relative root) "lib"
-    let bin         = Path.(relative root) "bin"
-    let sbin        = Path.(relative root) "sbin"
-    let toplevel    = Path.(relative root) "lib/toplevel"
-    let share       = Path.(relative root) "share"
-    let share_root  = Path.(relative root) "share_root"
-    let etc         = Path.(relative root) "etc"
-    let doc         = Path.(relative root) "doc"
-    let stublibs    = Path.(relative root) "lib/stublibs"
-    let man         = Path.(relative root) "man"
-  end
-
   let relative_installed_path t ~package =
-    let main_dir =
-      match t.section with
-      | Bin        -> Paths.bin
-      | Sbin       -> Paths.sbin
-      | Toplevel   -> Paths.toplevel
-      | Share_root -> Paths.share_root
-      | Stublibs   -> Paths.stublibs
-      | Man        -> Paths.man
-      | Lib        -> Path.relative Paths.lib     package
-      | Libexec    -> Path.relative Paths.libexec package
-      | Share      -> Path.relative Paths.share   package
-      | Etc        -> Path.relative Paths.etc     package
-      | Doc        -> Path.relative Paths.doc     package
-      | Misc       -> invalid_arg "Install.Entry.relative_installed_path"
-    in
+    let main_dir = Section.install_dir t.section ~package in
     let dst =
       match t.dst with
       | Some x -> x
@@ -126,6 +127,18 @@ module Entry = struct
         | _ -> dst
     in
     Path.relative main_dir dst
+
+  let add_install_prefix t ~package ~prefix =
+    let opam_will_install_in_this_dir =
+      Section.install_dir t.section ~package
+    in
+    let i_want_to_install_the_file_as =
+      Path.append prefix (relative_installed_path t ~package)
+    in
+    let dst =
+      Path.reach i_want_to_install_the_file_as ~from:opam_will_install_in_this_dir
+    in
+    { t with dst = Some dst }
 end
 
 module SMap = Map.Make(Section)
diff --git a/src/install.mli b/src/install.mli
index deb30a63..be64cf63 100644
--- a/src/install.mli
+++ b/src/install.mli
@@ -29,6 +29,7 @@ module Entry : sig
   val set_src : t -> Path.t -> t
 
   val relative_installed_path : t -> package:string -> Path.t
+  val add_install_prefix : t -> package:string -> prefix:Path.t -> t
 end
 
 val files : Entry.t list -> Path.Set.t