diff --git a/doc/scripts/subst.sh b/doc/scripts/subst.sh
index 0a9909c..698d3dd 100755
--- a/doc/scripts/subst.sh
+++ b/doc/scripts/subst.sh
@@ -37,29 +37,6 @@ MANDIR=
 PROGNAME="subst.sh"
 SYSCONFDIR=
 [ -f "$CONFIGSH" ] && . "$CONFIGSH"
-[ -z "$BINDIR" ] && BINDIR="$PREFIX/bin"
-[ -z "$DATADIR" ] && DATADIR="$PREFIX/share"
-[ -z "$INCLUDEDIR" ] && INCLUDEDIR="$PREFIX/include"
-if [ -z "$LDSO" ]; then
-	case "$(uname -s)" in
-		FreeBSD)
-			LDSO="/libexec/ld-elf.so.1"
-			;;
-		Linux)
-			LDSO="/lib/ld-linux-$(uname -p).so.2"
-			;;
-		*)
-			LDSO="/libexec/ld.elf_so"
-			;;
-	esac
-fi
-[ -z "$LIBDIR" ] && LIBDIR="$PREFIX/lib"
-[ -z "$LIBEXECDIR" ] && LIBEXECDIR="$PREFIX/libexec"
-[ -z "$MANDIR" ] && MANDIR="$DATADIR/man"
-if [ -z "$SYSCONFDIR" ]; then
-	SYSCONFDIR="$PREFIX/etc"
-	[ "$PREFIX" = "/usr" ] && SYSCONFDIR="/etc"
-fi
 #executables
 CHMOD="chmod"
 DEBUG="_debug"
@@ -71,6 +48,92 @@ SED="sed"
 
 
 #functions
+#subst
+_subst()
+{
+	#check the variables
+	if [ -z "$PACKAGE" ]; then
+		_error "The PACKAGE variable needs to be set"
+		return $?
+	fi
+	if [ -z "$VERSION" ]; then
+		_error "The VERSION variable needs to be set"
+		return $?
+	fi
+	[ -z "$BINDIR" ] && BINDIR="$PREFIX/bin"
+	[ -z "$DATADIR" ] && DATADIR="$PREFIX/share"
+	[ -z "$INCLUDEDIR" ] && INCLUDEDIR="$PREFIX/include"
+	if [ -z "$LDSO" ]; then
+		case "$(uname -s)" in
+			FreeBSD)
+				LDSO="/libexec/ld-elf.so.1"
+			;;
+			Linux)
+				LDSO="/lib/ld-linux-$(uname -p).so.2"
+				;;
+			*)
+				LDSO="/libexec/ld.elf_so"
+				;;
+		esac
+	fi
+	[ -z "$LIBDIR" ] && LIBDIR="$PREFIX/lib"
+	[ -z "$LIBEXECDIR" ] && LIBEXECDIR="$PREFIX/libexec"
+	[ -z "$MANDIR" ] && MANDIR="$DATADIR/man"
+	if [ -z "$SYSCONFDIR" ]; then
+		SYSCONFDIR="$PREFIX/etc"
+		[ "$PREFIX" = "/usr" ] && SYSCONFDIR="/etc"
+	fi
+
+	while [ $# -gt 0 ]; do
+		target="$1"
+		shift
+
+		#clean
+		[ "$clean" -ne 0 ] && continue
+
+		#uninstall
+		if [ "$uninstall" -eq 1 ]; then
+			$DEBUG $RM -- "$PREFIX/$target"		|| return 2
+			continue
+		fi
+
+		#install
+		if [ "$install" -eq 1 ]; then
+			source="${target#$OBJDIR}"
+			$DEBUG $MKDIR -- "$PREFIX"		|| return 2
+			mode="-m 0644"
+			[ -x "${source}.in" ] && mode="-m 0755"
+			$DEBUG $INSTALL $mode "$target" "$PREFIX/$source" \
+								|| return 2
+			continue
+		fi
+
+		#create
+		source="${target#$OBJDIR}"
+		source="${source}.in"
+		$DEBUG $SED -e "s,@PACKAGE@,$PACKAGE,g" \
+			-e "s,@VERSION@,$VERSION,g" \
+			-e "s,@PREFIX@,$PREFIX,g" \
+			-e "s,@BINDIR@,$BINDIR,g" \
+			-e "s,@DATADIR@,$DATADIR,g" \
+			-e "s,@INCLUDEDIR@,$INCLUDEDIR,g" \
+			-e "s,@LDSO@,$LDSO,g" \
+			-e "s,@LIBDIR@,$LIBDIR,g" \
+			-e "s,@LIBEXECDIR@,$LIBEXECDIR,g" \
+			-e "s,@MANDIR@,$MANDIR,g" \
+			-e "s,@SYSCONFDIR@,$SYSCONFDIR,g" \
+			-e "s,@PWD@,$PWD,g" \
+			-- "$source" > "$target"
+		if [ $? -ne 0 ]; then
+			$RM -- "$target" 2> "$DEVNULL"
+			return 2
+		elif [ -x "$source" ]; then
+			$DEBUG $CHMOD -- 0755 "$target"
+		fi
+	done
+	return 0
+}
+
 #debug
 _debug()
 {
@@ -99,7 +162,7 @@ _usage()
 clean=0
 install=0
 uninstall=0
-while getopts "ciuP:" name; do
+while getopts "ciuO:P:" name; do
 	case $name in
 		c)
 			clean=1
@@ -112,6 +175,9 @@ while getopts "ciuP:" name; do
 			install=0
 			uninstall=1
 			;;
+		O)
+			export "${OPTARG%%=*}"="${OPTARG#*=}"
+			;;
 		P)
 			PREFIX="$OPTARG"
 			;;
@@ -127,61 +193,5 @@ if [ $# -eq 0 ]; then
 	exit $?
 fi
 
-#check the variables
-if [ -z "$PACKAGE" ]; then
-	_error "The PACKAGE variable needs to be set"
-	exit $?
-fi
-if [ -z "$VERSION" ]; then
-	_error "The VERSION variable needs to be set"
-	exit $?
-fi
-
 exec 3>&1
-while [ $# -gt 0 ]; do
-	target="$1"
-	shift
-
-	#clean
-	[ "$clean" -ne 0 ] && continue
-
-	#uninstall
-	if [ "$uninstall" -eq 1 ]; then
-		$DEBUG $RM -- "$PREFIX/$target"			|| exit 2
-		continue
-	fi
-
-	#install
-	if [ "$install" -eq 1 ]; then
-		source="${target#$OBJDIR}"
-		$DEBUG $MKDIR -- "$PREFIX"			|| exit 2
-		mode="-m 0644"
-		[ -x "${source}.in" ] && mode="-m 0755"
-		$DEBUG $INSTALL $mode "$target" "$PREFIX/$source" \
-								|| exit 2
-		continue
-	fi
-
-	#create
-	source="${target#$OBJDIR}"
-	source="${source}.in"
-	$DEBUG $SED -e "s,@PACKAGE@,$PACKAGE,g" \
-		-e "s,@VERSION@,$VERSION,g" \
-		-e "s,@PREFIX@,$PREFIX,g" \
-		-e "s,@BINDIR@,$BINDIR,g" \
-		-e "s,@DATADIR@,$DATADIR,g" \
-		-e "s,@INCLUDEDIR@,$INCLUDEDIR,g" \
-		-e "s,@LDSO@,$LDSO,g" \
-		-e "s,@LIBDIR@,$LIBDIR,g" \
-		-e "s,@LIBEXECDIR@,$LIBEXECDIR,g" \
-		-e "s,@MANDIR@,$MANDIR,g" \
-		-e "s,@SYSCONFDIR@,$SYSCONFDIR,g" \
-		-e "s,@PWD@,$PWD,g" \
-		-- "$source" > "$target"
-	if [ $? -ne 0 ]; then
-		$RM -- "$target" 2> "$DEVNULL"
-		exit 2
-	elif [ -x "$source" ]; then
-		$DEBUG $CHMOD -- 0755 "$target"
-	fi
-done
+_subst "$@"