diff --git a/slicer/slice.c b/slicer/slice.c
new file mode 100644
index 0000000..8022edd
--- /dev/null
+++ b/slicer/slice.c
@@ -0,0 +1,150 @@
+/*
+ * slice.c - Generate slices
+ *
+ * Written 2015 by Werner Almesberger
+ * Copyright 2015 by 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.
+ */
+
+
+#include <stdbool.h>
+#include <math.h>
+#include <glib.h>
+
+#include "util.h"
+#include "stl.h"
+#include "slice.h"
+
+
+struct line {
+	float ax, ay;
+	float bx, by;
+	struct line *next;
+};
+
+
+struct z {
+	float z;
+	bool present;
+	struct line *lines;
+};
+
+
+static GTree *tree;
+
+
+static inline bool eq(float a, float b)
+{
+	return fabsf(a - b) < 1e-6;
+}
+
+
+static struct z *get_z(float z)
+{
+	struct z *base;
+
+	base = g_tree_lookup(tree, &z);
+	if (base)
+		return base;
+	base = alloc_type(struct z);
+	base->z = z;
+	base->present = 0;
+	base->lines = NULL;
+	g_tree_insert(tree, &base->z, base);
+	return base;
+}
+
+
+static void mark_z(float z)
+{
+	struct z *base;
+
+	base = get_z(z);
+	base->present = 1;
+}
+
+
+static void add(const struct v *a, const struct v *b, const struct v *c)
+{
+	struct z *z;
+	struct line *line;
+
+	/* @@@ should check that C is above AB */
+
+	if (eq(a->x, b->x) && eq(a->y, b->y)) {
+fprintf(stderr, "zero point\n");
+		return;
+	}
+	z = get_z(a->z);
+	line = alloc_type(struct line);
+	line->ax = a->x;
+	line->ay = a->y;
+	line->bx = b->x;
+	line->by = b->y;
+	line->next = z->lines;
+	z->lines = line;
+}
+
+
+void slice(struct v f[3])
+{
+	bool ab = eq(f[0].z, f[1].z);
+	bool bc = eq(f[1].z, f[2].z);
+	bool ac = eq(f[0].z, f[2].z);
+
+	if (ab && bc)
+		mark_z(f[0].z);
+	else if (ab)
+		add(f + 0, f + 1, f + 2);
+	else if (bc)
+		add(f + 1, f + 2, f + 0);
+	else if (ac)
+		add(f + 0, f + 2, f + 1);
+}
+
+
+/* ----- Dumping ----------------------------------------------------------- */
+
+
+static gboolean dump(gpointer key, gpointer value, gpointer data)
+{
+	const struct z *z = value;
+	const struct line *line;
+
+	if (!z->present)
+		return 0;
+	for (line = z->lines; line; line = line->next)
+		printf("%f %f %f\n%f %f %f\n\n\n",
+		    line->ax, line->ay, z->z,
+		    line->bx, line->by, z->z);
+	return 0;
+}
+
+
+void slice_dump(void)
+{
+	g_tree_foreach(tree, dump, NULL);
+}
+
+
+/* ----- Initialization ---------------------------------------------------- */
+
+
+static gint comp(gconstpointer a, gconstpointer b)
+{
+	const float *za = a;
+	const float *zb = b;
+
+	return eq(*za, *zb) ? 0 : *za < *zb ? -1 : 1;
+
+}
+
+
+void slice_init(void)
+{
+	tree = g_tree_new(comp);
+}
diff --git a/slicer/slice.h b/slicer/slice.h
new file mode 100644
index 0000000..fbaa9ac
--- /dev/null
+++ b/slicer/slice.h
@@ -0,0 +1,24 @@
+/*
+ * slice.h - Generate slices
+ *
+ * Written 2015 by Werner Almesberger
+ * Copyright 2015 by 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.
+ */
+
+
+#ifndef SLICE_H
+#define	SLICE_H
+
+#include "stl.h"
+
+
+void slice(struct v f[3]);
+void slice_dump(void);
+void slice_init(void);
+
+#endif /* !SLICE_H */
diff --git a/slicer/util.h b/slicer/util.h
new file mode 100644
index 0000000..56f4b8b
--- /dev/null
+++ b/slicer/util.h
@@ -0,0 +1,30 @@
+/*
+ * util.h - Common utility functions
+ *
+ * Written 2006, 2009, 2010 by Werner Almesberger
+ * Copyright 2006, 2009, 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.
+ */
+
+
+#ifndef UTIL_H
+#define UTIL_H
+
+#include <stdarg.h>
+#include <stdlib.h>
+#include <string.h>
+
+
+#define alloc_size(s)					\
+    ({	void *alloc_size_tmp = malloc(s);		\
+	if (!alloc_size_tmp)				\
+		abort();				\
+	alloc_size_tmp; })
+
+#define alloc_type(t) ((t *) alloc_size(sizeof(t)))
+
+#endif /* !UTIL_H */