diff --git a/builder/builder.ml b/builder/builder.ml
index 97a9f0d37..d8e625f68 100644
--- a/builder/builder.ml
+++ b/builder/builder.ml
@@ -396,18 +396,15 @@ let main () =
         (human_size size) output_filename (human_size blockdev_size);
     size in
 
-  let goal =
-    (* MUST *)
-    let goal_must = [
-      `Filename, output_filename;
-      `Size, Int64.to_string output_size;
-      `Format, output_format
-    ] in
+  (* Goal: must *)
+  let must = [
+    `Filename, output_filename;
+    `Size, Int64.to_string output_size;
+    `Format, output_format
+  ] in
 
-    (* MUST NOT *)
-    let goal_must_not = [ `Template, ""; `XZ, "" ] in
-
-    goal_must, goal_must_not in
+  (* Goal: must not *)
+  let must_not = [ `Template, ""; `XZ, "" ] in
 
   let cache_dir = (open_guestfs ())#get_cachedir () in
 
@@ -508,7 +505,7 @@ let main () =
   (* Plan how to create the disk image. *)
   message (f_"Planning how to build this image");
   let plan =
-    try plan ~max_depth:5 transitions itags goal
+    try plan ~max_depth:5 transitions itags ~must ~must_not
     with
       Failure "plan" ->
         error (f_"no plan could be found for making a disk image with\nthe required size, format etc. This is a bug in libguestfs!\nPlease file a bug, giving the command line arguments you used.");
diff --git a/common/mltools/planner.ml b/common/mltools/planner.ml
index 12dfb267c..736cfee92 100644
--- a/common/mltools/planner.ml
+++ b/common/mltools/planner.ml
@@ -20,13 +20,15 @@ type ('name, 'value) tag = 'name * 'value
 
 type ('name, 'value) tags = ('name, 'value) tag list
 
-type ('name, 'value, 'task) plan =
-  (('name, 'value) tags * 'task * ('name, 'value) tags) list
+type ('name, 'value, 'task) transition =
+  ('name, 'value) tags * 'task * ('name, 'value) tags
+
+type ('name, 'value, 'task) plan = ('name, 'value, 'task) transition list
 
 type ('name, 'value, 'task) transitions_function =
   ('name, 'value) tags -> ('task * int * ('name, 'value) tags) list
 
-let plan ?(max_depth = 10) transitions itags (goal_must, goal_must_not) =
+let plan ?(max_depth = 10) transitions itags ~must ~must_not =
   (* Do the given output tags match the finish condition? *)
   let finished (otags, _, _) =
     let must =
@@ -34,14 +36,14 @@ let plan ?(max_depth = 10) transitions itags (goal_must, goal_must_not) =
       List.for_all (
         fun (name, value) ->
           try List.assoc name otags = value with Not_found -> false
-      ) goal_must in
+      ) must in
 
     let must_not =
       (* No tag from the MUST NOT list can appear. *)
       List.for_all (
         fun (name, value) ->
           try List.assoc name otags <> value with Not_found -> true
-      ) goal_must_not in
+      ) must_not in
 
     must && must_not
   in
diff --git a/common/mltools/planner.mli b/common/mltools/planner.mli
index 6b5a12d55..8cd1c51c9 100644
--- a/common/mltools/planner.mli
+++ b/common/mltools/planner.mli
@@ -41,24 +41,41 @@
     possible, but might not be optimal. *)
 
 type ('name, 'value) tag = 'name * 'value
+(** A single tag. *)
 
 type ('name, 'value) tags = ('name, 'value) tag list
-  (** An assoc-list of tags. *)
+(** An assoc-list of tags. *)
 
-type ('name, 'value, 'task) plan =
-  (('name, 'value) tags * 'task * ('name, 'value) tags) list
+type ('name, 'value, 'task) transition =
+  ('name, 'value) tags * 'task * ('name, 'value) tags
+(** A transition is the 3 element tuple:
+    (input tags for this transition,
+     the task to perform,
+     the output tags after this transition). *)
+
+type ('name, 'value, 'task) plan = ('name, 'value, 'task) transition list
+(** A plan (the returned value from the {!plan} function) is a list
+    of transitions. *)
 
 type ('name, 'value, 'task) transitions_function =
   ('name, 'value) tags -> ('task * int * ('name, 'value) tags) list
+(** This is the type of the transition function.  Given a set of
+    current tags, it returns the list of possible transitions out,
+    with a weight (higher number = more expensive) for each and the
+    resulting set of tags after that transition. *)
 
-val plan : ?max_depth:int -> ('name, 'value, 'task) transitions_function -> ('name, 'value) tags -> ('name, 'value) tags * ('name, 'value) tags -> ('name, 'value, 'task) plan
+val plan : ?max_depth:int -> ('name, 'value, 'task) transitions_function ->
+           ('name, 'value) tags ->
+           must: ('name, 'value) tags -> must_not: ('name, 'value) tags ->
+           ('name, 'value, 'task) plan
 (** Make a plan.
 
-    [plan transitions itags (goal_must, goal_must_not)] works out a
+    [plan transitions itags goal_must goal_must_not] works out a
     plan, which is a list of tasks that have to be carried out in
-    order to go from the input tags to the goal.  The goal is passed
-    in as a pair of lists: tags that MUST appear and tags that MUST
-    NOT appear.
+    order to go from the input tags to the goal.
+
+    The goal is passed in as a pair of lists: tags that MUST appear
+    and tags that MUST NOT appear.
 
     The returned value is a {!plan}.