diff --git a/src/lib.c b/src/lib.c
index f8925536..2be271f3 100644
--- a/src/lib.c
+++ b/src/lib.c
@@ -173,3 +173,46 @@ integer_to_char (struct scm *x)
 {
   return make_char (x->value);
 }
+
+/*
+void
+assert_type (long type, char const *name_name, struct scm *x)
+{
+  if (x->type != type)
+    {
+      eputs (name);
+      eputs (": ");
+      error (cell_wrong_type_arg, cons (x, cell_nil));
+    }
+}
+*/
+
+void
+assert_num (long pos, struct scm *x)
+{
+  if (x->type != TNUMBER)
+    error (cell_symbol_wrong_type_arg, cons (cell_type_number, cons (make_number (pos), x)));
+}
+
+void
+assert_struct (long pos, struct scm *x)
+{
+  if (x->type != TSTRUCT)
+    error (cell_symbol_wrong_type_arg, cons (cell_type_struct, cons (make_number (pos), x)));
+}
+
+void
+assert_range (int assert, long i)
+{
+  if (assert == 0)
+    {
+      eputs ("value out of range: ");
+      eputs (ltoa (i));
+      eputs (": ");
+      assert_msg (assert, "value out of range");
+    }
+  /*
+  if (assert != 0)
+    error (cell_symbol_out_of_range, cons (cell_type_struct, cons (make_number (pos), cons (x, cell_nil))));
+  */
+}
diff --git a/src/struct.c b/src/struct.c
index 19162c70..540bd67d 100644
--- a/src/struct.c
+++ b/src/struct.c
@@ -54,11 +54,15 @@ struct_length (struct scm *x)
   return make_number (x->length);
 }
 
+void assert_num (long pos, struct scm *x);
+void assert_struct (long pos, struct scm *x);
+void assert_range (int assert, long x);
+
 struct scm *
 struct_ref_ (struct scm *x, long i)
 {
-  assert_msg (x->type == TSTRUCT, "x->type == TSTRUCT");
-  assert_msg (i < x->length, "i < x->length");
+  assert_struct (1, x);
+  assert_range (i < x->length, i);
   struct scm *e = cell_ref (x->structure, i);
   if (e->type == TREF)
     e = e->ref;