diff --git a/Makefile b/Makefile
index 202c577..4644d56 100644
--- a/Makefile
+++ b/Makefile
@@ -8,7 +8,8 @@ DOC = cpdfunicodedata cpdferror cpdfdebug cpdfjson cpdfstrftime cpdfcoord \
       cpdfsqueeze cpdfdraft cpdfspot cpdfpagelabels cpdfcreate cpdfannot \
       cpdfxobject cpdfimpose cpdfchop cpdftweak cpdftexttopdf cpdftoc \
       cpdfjpeg cpdfjpeg2000 cpdfpng cpdfimage cpdfdraw cpdfcomposition \
-      cpdfshape cpdfcolours cpdfdrawcontrol cpdfprinttree cpdfua cpdfcommand
+      cpdfshape cpdfcolours cpdfdrawcontrol cpdfprinttree cpdfst cpdfua \
+      cpdfcommand
 
 MODS = $(NONDOC) $(DOC)
 
diff --git a/cpdfst.ml b/cpdfst.ml
new file mode 100644
index 0000000..4c0f67a
--- /dev/null
+++ b/cpdfst.ml
@@ -0,0 +1 @@
+(* Operations on structure trees. *)
diff --git a/cpdfst.mli b/cpdfst.mli
new file mode 100644
index 0000000..d24b711
--- /dev/null
+++ b/cpdfst.mli
@@ -0,0 +1 @@
+(** Structure trees *)