diff --git a/cpdfst.ml b/cpdfst.ml
index 4c0f67a..5631cb2 100644
--- a/cpdfst.ml
+++ b/cpdfst.ml
@@ -1 +1,4 @@
 (* Operations on structure trees. *)
+open Pdfutil
+
+let trim_structure_tree pdf range = ()
diff --git a/cpdfst.mli b/cpdfst.mli
index d24b711..a0ac2d0 100644
--- a/cpdfst.mli
+++ b/cpdfst.mli
@@ -1 +1,4 @@
 (** Structure trees *)
+
+(** Trim structure tree to remove parts marked as not in the page range given. *)
+val trim_structure_tree : Pdf.t -> int list -> unit