diff --git a/Makefile b/Makefile
index 30ff3ca..17c4be9 100644
--- a/Makefile
+++ b/Makefile
@@ -82,7 +82,7 @@ endif
 # ----- Rules -----------------------------------------------------------------
 
 .PHONY:		all dep depend clean install uninstall manual upload-manual
-.PHONY:		update montage
+.PHONY:		update montage test tests valgrind
 
 .SUFFIXES:	.fig .xpm .ppm
 
@@ -156,6 +156,18 @@ dep depend .depend:
 
 -include $(OBJS:.o=.d)
 
+# ----- Tests -----------------------------------------------------------------
+
+test tests:	all
+		LANG= sh -c \
+		  'passed=0 && cd test && \
+		  for n in [a-z]*; do \
+		  SCRIPT=$$n . ./$$n; done; \
+		  echo "Passed all $$passed tests"'
+
+valgrind:
+		VALGRIND="valgrind -q" $(MAKE) tests
+
 # ----- Cleanup ---------------------------------------------------------------
 
 clean:
diff --git a/README b/README
index 294d712..8de9a81 100644
--- a/README
+++ b/README
@@ -613,4 +613,4 @@ output. %exit immediately exits fped, without invoking the GUI.
 %tsort is used to test-drive the topological sort algorithm. The items
 in the curly braces are declarations of nodes with (-<id>) or without
 (+<id>) decay or edges in the partial order. The optional number is
-the edge's priority. See tsort.c for details.
+the edge's priority. See tsort.c for details, test/tsort for examples.
diff --git a/fped.c b/fped.c
index dc8d70d..9b6c353 100644
--- a/fped.c
+++ b/fped.c
@@ -65,10 +65,11 @@ static void load_file(const char *name)
 static void usage(const char *name)
 {
 	fprintf(stderr,
-"usage: %s [-k] [-p|-P] [cpp_option ...] [in_file [out_file]]\n\n"
+"usage: %s [-k] [-p|-P] [-T] [cpp_option ...] [in_file [out_file]]\n\n"
 "  -k          write KiCad output, then exit\n"
 "  -p          write Postscript output, then exit\n"
 "  -P          write Postscript output (full page), then exit\n"
+"  -T          test mode. Load file, then exit\n"
 "  cpp_option  -Idir, -Dname[=value], or -Uname\n"
     , name);
 	exit(1);
@@ -82,12 +83,13 @@ int main(int argc, char **argv)
 	char *args[2];
 	int fake_argc;
 	char opt[] = "-?";
-	int error, batch;
+	int error;
+	int batch = 0;
 	int batch_write_kicad = 0;
 	int batch_write_ps = 0, batch_write_ps_fullpage = 0;
 	int c;
 
-	while ((c = getopt(argc, argv, "kpD:I:U:P")) != EOF)
+	while ((c = getopt(argc, argv, "kpD:I:PTU:")) != EOF)
 		switch (c) {
 		case 'k':
 			batch_write_kicad = 1;
@@ -98,6 +100,9 @@ int main(int argc, char **argv)
 		case 'P':
 			batch_write_ps_fullpage = 1;
 			break;
+		case 'T':
+			batch = 1;
+			break;
 		case 'D':
 		case 'U':
 		case 'I':
@@ -112,7 +117,8 @@ int main(int argc, char **argv)
 	if (batch_write_ps && batch_write_ps_fullpage)
 		usage(name);
 
-	batch = batch_write_kicad || batch_write_ps || batch_write_ps_fullpage;
+	if (batch_write_kicad || batch_write_ps || batch_write_ps_fullpage)
+		batch = 1;
 
 	if (!batch) {
 		args[0] = name;
diff --git a/test/Common b/test/Common
new file mode 100755
index 0000000..42989eb
--- /dev/null
+++ b/test/Common
@@ -0,0 +1,56 @@
+#!/bin/sh
+#
+# Common - Elements shared by all regression tests for fped
+#
+# Written 2010 by Werner Almesberger
+# Copyright 2010 Werner Almesberger
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+
+
+fped()
+{
+    echo -n "$1: " 1>&2
+    shift
+    cat >_in
+    $VALGRIND ../fped -T _in "$@" >_out 2>&1 || {
+	echo FAILED "($SCRIPT)" 1>&2
+	cat _out
+	rm -f _in _out
+	exit 1
+    }
+    rm -f _in
+}
+
+
+fped_fail()
+{
+    echo -n "$1: " 1>&2
+    shift
+    cat >_in
+    $VALGRIND ../fped -T _in "$@" >_out 2>&1 && {
+	echo FAILED "($SCRIPT)" 1>&2
+	cat _out
+	rm -f _in _out
+	exit 1
+    }
+    rm -f _in
+}
+
+
+expect()
+{
+    diff -u - "$@" _out >_diff || {
+	echo FAILED "($SCRIPT)" 1>&2
+	cat _diff 1>&2
+	rm -f _out _diff
+	exit 1
+    }
+    echo PASSED 1>&2
+    rm -f _out _diff
+    passed=`expr ${passed:-0} + 1`
+}
diff --git a/test/tsort b/test/tsort
new file mode 100644
index 0000000..52806be
--- /dev/null
+++ b/test/tsort
@@ -0,0 +1,155 @@
+#!/bin/sh
+. ./Common
+
+###############################################################################
+
+fped "tsort: total order" <<EOF
+package "_"
+
+%tsort {
+	a b
+	a c
+	a d
+	b c
+	b d
+	c d
+}
+EOF
+expect <<EOF
+a
+b
+c
+d
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "tsort: partial order change (1)" <<EOF
+package "_"
+
+%tsort {
+	a b
+	a c
+	a d
+	d b
+}
+EOF
+expect <<EOF
+a
+c
+d
+b
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "tsort: partial order change (2)" <<EOF
+package "_"
+
+%tsort {
+	b c
+	c d
+	a b
+}
+EOF
+expect <<EOF
+a
+b
+c
+d
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "tsort: old order differs from resolution order" <<EOF
+package "_"
+
+%tsort {
+	+a +b +c +d
+	a c
+	a b
+	a d
+}
+EOF
+expect <<EOF
+a
+b
+c
+d
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "tsort: order change due to priority" <<EOF
+package "_"
+
+%tsort {
+	a b
+	a c 1
+	a d
+}
+EOF
+expect <<EOF
+a
+c
+b
+d
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "tsort: priority accumulation without decay" <<EOF
+package "_"
+
+%tsort {
+	+a +b +c +d
+	a b 1
+	a d 1
+}
+EOF
+expect <<EOF
+a
+b
+d
+c
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "tsort: priority accumulation with decay" <<EOF
+package "_"
+
+%tsort {
+	+a -b +c +d
+	a b 1
+	a d 1
+}
+EOF
+expect <<EOF
+a
+b
+c
+d
+EOF
+
+#------------------------------------------------------------------------------
+
+fped_fail "tsort: cycle" <<EOF
+package "_"
+
+%tsort {
+	a b
+	b a
+}
+EOF
+expect <<EOF
+cycle detected in partial order
+Aborted (core dumped)
+EOF
+
+# not entirely comfortable about the "Aborted (core dumped)". It's a system
+# message (from the shell) that may get mangled. Also, since few people keep
+# their cores these days, "(core dumped)" shouldn't really appear. Wonder why
+# it does. strace agrees that __WCOREFLAG is set ...
+
+###############################################################################
diff --git a/tsort.c b/tsort.c
index d54fe6e..1345ae5 100644
--- a/tsort.c
+++ b/tsort.c
@@ -152,8 +152,10 @@ void **end_tsort(struct tsort *tsort)
 		}
 		free(node);
 	}
-	if (tsort->nodes) /* we have at least one cycle */
+	if (tsort->nodes) {
+		fprintf(stderr, "cycle detected in partial order\n");
 		abort();
+	}
 	free(tsort);
 	res[n] = NULL;
 	return res;