diff --git a/doc/preprocess.sh b/doc/preprocess.sh
index 726ce0a..6ee75f2 100755
--- a/doc/preprocess.sh
+++ b/doc/preprocess.sh
@@ -1,4 +1,6 @@
 #! /usr/bin/env bash
 # Run GCC preprocessor and delete empty lines
 : ${CC:=cc}
+PS4='doc/preprocess.sh|# '
+set -x
 "${CC}" -E -DURI_DOXYGEN -DURI_NO_UNICODE -C -I ../include "$1" | sed -e '/^$/d' -e 's/COMMENT_HACK//g'