diff --git a/lib/StackAction.ml b/lib/StackAction.ml
index d6e0e85..b18c937 100644
--- a/lib/StackAction.ml
+++ b/lib/StackAction.ml
@@ -14,3 +14,5 @@ let merge lhs rhs =
   { pop = lhs.pop + pop; push = List.append push rhs.push; }
 
 let empty = { pop = 0; push = []; }
+
+let map f sta = { pop = sta.pop; push = List.map f sta.push; }
diff --git a/lib/StackAction.mli b/lib/StackAction.mli
new file mode 100644
index 0000000..3da10cd
--- /dev/null
+++ b/lib/StackAction.mli
@@ -0,0 +1,11 @@
+(** A stack action (pop is run first, then push sequentially) *)
+type 'a t =
+  { pop  : int
+  ; push : 'a list
+  }
+
+val merge : 'a t -> 'a t -> 'a t
+
+val empty : 'a t
+
+val map : ('a -> 'b) -> 'a t -> 'b t