diff --git a/Makefile b/Makefile
index 178b77f4e..d48623b49 100644
--- a/Makefile
+++ b/Makefile
@@ -31,6 +31,9 @@ TEST_CLI_PROGRAMS :=				\
 	test-cli/update-channel-htlc-remove	\
 	test-cli/update-channel-signature
 
+TEST_PROGRAMS :=				\
+	test/test_state_coverage
+
 BITCOIN_OBJS :=					\
 	bitcoin/address.o			\
 	bitcoin/base58.o			\
@@ -79,7 +82,7 @@ CCAN_EXTRA_OBJS :=				\
 
 CDUMP_OBJS := ccan-cdump.o ccan-strmap.o
 
-PROGRAMS := $(TEST_CLI_PROGRAMS)
+PROGRAMS := $(TEST_CLI_PROGRAMS) $(TEST_PROGRAMS)
 
 HEADERS := $(filter-out gen_*, $(wildcard *.h)) $(wildcard bitcoin/*.h) gen_state_names.h
 
@@ -109,6 +112,7 @@ lightning.pb-c.c lightning.pb-c.h: lightning.proto
 	$(PROTOCC) lightning.proto --c_out=.
 
 $(TEST_CLI_PROGRAMS): % : %.o $(HELPER_OBJS) $(BITCOIN_OBJS) $(CCAN_OBJS) libsecp256k1.a
+$(TEST_PROGRAMS): % : %.o $(BITCOIN_OBJS) $(CCAN_OBJS) $(CCAN_EXTRA_OBJS) libsecp256k1.a
 $(PROGRAMS:=.o) $(HELPER_OBJS): $(HEADERS)
 
 $(CCAN_OBJS) $(HELPER_OBJS) $(PROGRAM_OBJS) $(BITCOIN_OBJS) $(CDUMP_OBJS): ccan/config.h
diff --git a/state.c b/state.c
index 077404731..f626b3946 100644
--- a/state.c
+++ b/state.c
@@ -769,7 +769,7 @@ start_closing:
 	set_effect(effect, close_timeout, INPUT_CLOSE_COMPLETE_TIMEOUT);
 
 	set_effect(effect, watch,
-		   bitcoin_watch_close(effect, BITCOIN_CLOSE_DONE));
+		   bitcoin_watch_close(effect, sdata, BITCOIN_CLOSE_DONE));
 
 	/* As soon as we send packet, they could close. */
 	set_effect(effect, send, pkt_close(effect, sdata));
diff --git a/test/test_state_coverage.c b/test/test_state_coverage.c
new file mode 100644
index 000000000..8af08c865
--- /dev/null
+++ b/test/test_state_coverage.c
@@ -0,0 +1,872 @@
+/* Test for state machine. */
+#include <stdbool.h>
+#include <ccan/array_size/array_size.h>
+#include <ccan/tal/tal.h>
+#include <ccan/tal/str/str.h>
+#include <ccan/err/err.h>
+#include <ccan/structeq/structeq.h>
+#include <ccan/htable/htable_type.h>
+#include <ccan/hash/hash.h>
+#include "state.h"
+#include "gen_state_names.h"
+
+/* To recontruct errors. */
+struct trail {
+	struct trail *next;
+	const char *problem;
+	const char *name;
+	enum state_input input;
+	enum state before, after;
+	const char *pkt_sent;
+};
+
+struct state_data {
+	enum state state;
+	size_t num_outputs;
+	enum state_input outputs[6];
+	enum state_input current_command;
+	enum state_input deferred_pkt;
+	enum state deferred_state;
+	bool pkt_inputs;
+	bool cmd_inputs;
+
+	const char *error;
+	
+	/* What bitcoin/timeout notifications are we subscribed to? */
+	uint64_t event_notifies;
+	/* ID. */
+	const char *name;
+	/* The other peer's sdata. */
+	struct state_data *peer;
+};
+
+struct history {
+	struct state_data a, b;
+};
+
+static const struct history *history_keyof(const struct history *history)
+{
+	return history;
+}
+
+static uint32_t hash_add(uint64_t val, uint32_t base)
+{
+	return hash_any(&val, sizeof(val), base);
+}
+
+static size_t sdata_hash(const struct state_data *sdata)
+{
+	size_t h;
+
+	h = hash(sdata->outputs, sdata->num_outputs, sdata->state);
+	h = hash_add(sdata->current_command, h);
+	h = hash_add(sdata->deferred_pkt, h);
+	h = hash_add(sdata->deferred_state, h);
+	h = hash_add(sdata->pkt_inputs, h);
+	h = hash_add(sdata->event_notifies, h);
+
+	return h;
+}
+
+static bool sdata_eq(const struct state_data *a, const struct state_data *b)
+{
+	assert(streq(a->name, b->name));
+	return a->state == b->state
+		&& a->num_outputs == b->num_outputs
+		&& memcmp(a->outputs, b->outputs,
+			  a->num_outputs * sizeof(*a->outputs)) == 0
+		&& a->current_command == b->current_command
+		&& a->deferred_pkt == b->deferred_pkt
+		&& a->deferred_state == b->deferred_state
+		&& a->pkt_inputs == b->pkt_inputs
+		&& a->cmd_inputs == b->cmd_inputs
+		&& a->event_notifies == b->event_notifies;
+}
+
+static size_t history_hash(const struct history *history)
+{
+	return hash_add(sdata_hash(&history->a), sdata_hash(&history->b));
+}
+
+static bool history_eq(const struct history *a, const struct history *b)
+{
+	return sdata_eq(&a->a, &b->a) && sdata_eq(&a->b, &b->b);
+}
+
+HTABLE_DEFINE_TYPE(struct history, history_keyof, history_hash, history_eq, hist);
+
+static const char *state_name(enum state s)
+{
+	size_t i;
+
+	for (i = 0; enum_state_names[i].name; i++)
+		if (enum_state_names[i].v == s)
+			return enum_state_names[i].name;
+	return "unknown";
+}
+
+static const char *input_name(enum state_input in)
+{
+	size_t i;
+
+	for (i = 0; enum_state_input_names[i].name; i++)
+		if (enum_state_input_names[i].v == in)
+			return enum_state_input_names[i].name;
+	return "unknown";
+}
+
+static enum state_input input_by_name(const char *name)
+{
+	size_t i;
+
+	for (i = 0; enum_state_input_names[i].name; i++)
+		if (streq(name, enum_state_input_names[i].name))
+			return enum_state_input_names[i].v;
+	if (strstarts(name, "ERROR_PKT:"))
+		return PKT_ERROR;
+	abort();
+}
+
+static Pkt *new_pkt(const tal_t *ctx, enum state_input i)
+{
+	return (Pkt *)input_name(i);
+}
+	
+Pkt *pkt_open(const tal_t *ctx, const struct state_data *sdata)
+{
+	return new_pkt(ctx, PKT_OPEN);
+}
+
+Pkt *pkt_anchor(const tal_t *ctx, const struct state_data *sdata)
+{
+	return new_pkt(ctx, PKT_OPEN_ANCHOR);
+}
+
+Pkt *pkt_open_commit_sig(const tal_t *ctx, const struct state_data *sdata)
+{
+	return new_pkt(ctx, PKT_OPEN_COMMIT_SIG);
+}
+
+Pkt *pkt_open_complete(const tal_t *ctx, const struct state_data *sdata)
+{
+	return new_pkt(ctx, PKT_OPEN_COMPLETE);
+}
+
+Pkt *pkt_update(const tal_t *ctx, const struct state_data *sdata, void *data)
+{
+	return new_pkt(ctx, PKT_UPDATE);
+}
+		
+Pkt *pkt_htlc_update(const tal_t *ctx, const struct state_data *sdata, void *data)
+{
+	return new_pkt(ctx, PKT_UPDATE_ADD_HTLC);
+}
+
+Pkt *pkt_htlc_complete(const tal_t *ctx, const struct state_data *sdata, void *data)
+{
+	return new_pkt(ctx, PKT_UPDATE_COMPLETE_HTLC);
+}
+
+Pkt *pkt_htlc_timedout(const tal_t *ctx, const struct state_data *sdata, void *data)
+{
+	return new_pkt(ctx, PKT_UPDATE_TIMEDOUT_HTLC);
+}
+
+Pkt *pkt_htlc_routefail(const tal_t *ctx, const struct state_data *sdata, void *data)
+{
+	return new_pkt(ctx, PKT_UPDATE_ROUTEFAIL_HTLC);
+}
+
+Pkt *pkt_update_accept(const tal_t *ctx, const struct state_data *sdata)
+{
+	return new_pkt(ctx, PKT_UPDATE_ACCEPT);
+}
+
+Pkt *pkt_update_signature(const tal_t *ctx, const struct state_data *sdata)
+{
+	return new_pkt(ctx, PKT_UPDATE_SIGNATURE);
+}
+
+Pkt *pkt_update_complete(const tal_t *ctx, const struct state_data *sdata)
+{
+	return new_pkt(ctx, PKT_UPDATE_COMPLETE);
+}
+
+Pkt *pkt_err(const tal_t *ctx, const char *msg)
+{
+	return (Pkt *)tal_fmt(ctx, "ERROR_PKT:%s", msg);
+}
+
+Pkt *pkt_close(const tal_t *ctx, const struct state_data *sdata)
+{
+	return new_pkt(ctx, PKT_CLOSE);
+}
+
+Pkt *pkt_close_complete(const tal_t *ctx, const struct state_data *sdata)
+{
+	return new_pkt(ctx, PKT_CLOSE_COMPLETE);
+}
+
+Pkt *pkt_close_ack(const tal_t *ctx, const struct state_data *sdata)
+{
+	return new_pkt(ctx, PKT_CLOSE_ACK);
+}
+
+Pkt *unexpected_pkt(const tal_t *ctx, enum state_input input)
+{
+	return pkt_err(ctx, "Unexpected pkt");
+}
+
+Pkt *accept_pkt_open(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+
+Pkt *accept_pkt_anchor(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+
+Pkt *accept_pkt_open_commit_sig(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+	
+Pkt *accept_pkt_update(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+
+Pkt *accept_pkt_htlc_update(struct state_effect *effect,
+			    const struct state_data *sdata, const Pkt *pkt,
+			    Pkt **decline)
+{
+	/* FIXME: Test setting *decline. */
+	*decline = NULL;
+	return NULL;
+}
+
+Pkt *accept_pkt_htlc_routefail(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+
+Pkt *accept_pkt_htlc_timedout(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+
+Pkt *accept_pkt_htlc_complete(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+
+Pkt *accept_pkt_update_accept(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+
+Pkt *accept_pkt_update_complete(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+
+Pkt *accept_pkt_update_signature(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+
+Pkt *accept_pkt_close(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+
+Pkt *accept_pkt_close_complete(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+
+Pkt *accept_pkt_close_ack(struct state_effect *effect, const struct state_data *sdata, const Pkt *pkt)
+{
+	return NULL;
+}
+
+static struct bitcoin_tx *bitcoin_tx(const char *str)
+{
+	return (struct bitcoin_tx *)str;
+}
+
+static bool bitcoin_tx_is(const struct bitcoin_tx *btx, const char *str)
+{
+	return streq((const char *)btx, str);
+}
+
+struct bitcoin_tx *bitcoin_anchor(const tal_t *ctx,
+				  const struct state_data *sdata)
+{
+	return bitcoin_tx("anchor");
+}
+
+static void add_event(uint64_t *events, enum state_input input)
+{
+	/* This is how they say "no event please" */
+	if (input == INPUT_NONE)
+		return;
+			
+	assert(input < 64);
+	assert(!(*events & (1ULL << input)));
+	*events |= (1ULL << input);
+}
+
+struct watch {
+	uint64_t events;
+};
+
+struct watch *bitcoin_watch_anchor(const tal_t *ctx,
+				   const struct state_data *sdata,
+				   enum state_input depthok,
+				   enum state_input timeout,
+				   enum state_input unspent,
+				   enum state_input theyspent,
+				   enum state_input otherspent)
+{
+	struct watch *watch = talz(ctx, struct watch);
+
+	add_event(&watch->events, depthok);
+	add_event(&watch->events, timeout);
+	add_event(&watch->events, unspent);
+	add_event(&watch->events, theyspent);
+	add_event(&watch->events, otherspent);
+
+	/* We assume these values in activate_event. */
+	assert(timeout == BITCOIN_ANCHOR_TIMEOUT
+	       || timeout == INPUT_NONE);
+	assert(depthok == BITCOIN_ANCHOR_DEPTHOK);
+	return watch;
+}
+
+struct watch *bitcoin_unwatch_anchor_depth(const tal_t *ctx,
+					   const struct state_data *sdata,
+					   enum state_input depthok,
+					   enum state_input timeout)
+{
+	struct watch *watch = talz(ctx, struct watch);
+
+	add_event(&watch->events, depthok);
+	add_event(&watch->events, timeout);
+	return watch;
+}
+
+/* Wait for our commit to be spendable. */
+struct watch *bitcoin_watch_delayed(const struct state_effect *effect,
+				    enum state_input canspend)
+{
+	struct watch *watch = talz(effect, struct watch);
+
+	assert(bitcoin_tx_is(effect->broadcast, "our commit"));
+	add_event(&watch->events, canspend);
+	return watch;
+}
+
+/* Wait for commit to be very deeply buried (so we no longer need to
+ * even watch) */
+struct watch *bitcoin_watch(const struct state_effect *effect,
+			    enum state_input done)
+{
+	struct watch *watch = talz(effect, struct watch);
+
+	if (done == BITCOIN_STEAL_DONE)
+		assert(bitcoin_tx_is(effect->broadcast, "steal"));
+	else if (done == BITCOIN_SPEND_THEIRS_DONE)
+		assert(bitcoin_tx_is(effect->broadcast, "spend their commit"));
+	else if (done == BITCOIN_SPEND_OURS_DONE)
+		assert(bitcoin_tx_is(effect->broadcast, "spend our commit"));
+	else
+		errx(1, "Unknown watch effect %s", input_name(done));
+	add_event(&watch->events, done);
+	return watch;
+}
+
+/* Other side should drop close tx; watch for it. */
+struct watch *bitcoin_watch_close(const tal_t *ctx,
+				  const struct state_data *sdata,
+				  enum state_input done)
+{
+	struct watch *watch = talz(ctx, struct watch);
+	add_event(&watch->events, done);
+	return watch;
+}
+	
+struct bitcoin_tx *bitcoin_close(const tal_t *ctx,
+				 const struct state_data *sdata)
+{
+	return bitcoin_tx("close");
+}
+
+struct bitcoin_tx *bitcoin_spend_ours(const tal_t *ctx,
+				      const struct state_data *sdata)
+{
+	return bitcoin_tx("spend our commit");
+}
+
+struct bitcoin_tx *bitcoin_spend_theirs(const tal_t *ctx,
+					const struct state_data *sdata)
+{
+	return bitcoin_tx("spend their commit");
+}
+
+struct bitcoin_tx *bitcoin_steal(const tal_t *ctx,
+				 const struct state_data *sdata,
+					struct bitcoin_event *btc)
+{
+	/* FIXME: Test this failing! */
+	return bitcoin_tx("steal");
+}
+
+struct bitcoin_tx *bitcoin_commit(const tal_t *ctx,
+				  const struct state_data *sdata)
+{
+	return bitcoin_tx("our commit");
+}
+
+#include "state.c"
+#include <ccan/tal/tal.h>
+#include <stdio.h>
+
+static void sdata_init(struct state_data *sdata,
+		       struct state_data *other,
+		       enum state_input initstate,
+		       const char *name)
+{
+	sdata->state = initstate;
+	sdata->num_outputs = 1;
+	sdata->error = NULL;
+	memset(sdata->outputs, 0, sizeof(sdata->outputs));
+	sdata->deferred_pkt = INPUT_NONE;
+	sdata->deferred_state = STATE_MAX;
+	sdata->outputs[0] = INPUT_NONE;
+	sdata->current_command = INPUT_NONE;
+	sdata->event_notifies = 0;
+	sdata->pkt_inputs = true;
+	sdata->cmd_inputs = true;
+	sdata->name = name;
+	sdata->peer = other;
+}
+
+static void copy_peers(struct state_data *dst, struct state_data *peer,
+		       const struct state_data *src)
+{
+	*dst = *src;
+	*peer = *src->peer;
+	dst->peer = peer;
+	peer->peer = dst;
+}
+
+static bool visited_state(const struct hist *hist, enum state state, bool b)
+{
+	struct history *h;
+	struct hist_iter i;
+
+	for (h = hist_first(hist, &i); h; h = hist_next(hist, &i)) {
+		if (b) {
+			if (h->b.state == state)
+				return true;
+		} else {
+			if (h->a.state == state)
+				return true;
+		}
+	}
+	return false;
+}
+
+/* Recursion! */
+static struct trail *run_peer(const struct state_data *sdata,
+			      struct hist *hist);
+
+static bool hist_update(struct hist *hist, const struct state_data *sdata)
+{
+	struct history v;
+
+	if (streq(sdata->name, "A")) {
+		v.a = *sdata;
+		v.b = *sdata->peer;
+	} else {
+		v.b = *sdata;
+		v.a = *sdata->peer;
+	}
+
+	if (hist_get(hist, &v))
+		return false;
+
+	hist_add(hist, tal_dup(hist, struct history, &v));
+	return true;
+}
+
+static struct trail *add_trail(enum state_input input,
+			       const struct state_data *before,
+			       enum state after,
+			       const struct state_effect *effects,
+			       struct trail *next)
+{
+	struct trail *t = tal(NULL, struct trail);
+
+	t->name = before->name;
+	t->problem = next ? next->problem : NULL;
+	t->next = tal_steal(t, next);
+	t->input = input;
+	t->before = before->state;
+	t->after = after;
+	t->pkt_sent = (const char *)effects->send;
+	return t;
+}
+
+static struct trail *new_trail(enum state_input input,
+			       const struct state_data *before,
+			       enum state after,
+			       const struct state_effect *effects,
+			       const char *problem)
+{
+	struct trail *t = add_trail(input, before, after, effects, NULL);
+	t->problem = problem;
+	return t;
+}
+
+static bool is_current_command(const struct state_data *sdata,
+			       enum state_input cmd)
+{
+	if (cmd == CMD_SEND_UPDATE_ANY) {
+		return is_current_command(sdata, CMD_SEND_UPDATE)
+			|| is_current_command(sdata, CMD_SEND_HTLC_UPDATE)
+			|| is_current_command(sdata, CMD_SEND_HTLC_COMPLETE)
+			|| is_current_command(sdata, CMD_SEND_HTLC_TIMEDOUT)
+			|| is_current_command(sdata, CMD_SEND_HTLC_ROUTEFAIL);
+	}
+	return sdata->current_command == cmd;
+}
+
+static const char *apply_effects(struct state_data *sdata,
+				 const struct state_effect *effect)
+{
+	if (effect->send) {
+		const char *pkt = (const char *)effect->send;
+
+		/* Check for errors. */
+		if (strstarts(pkt, "ERROR_PKT:")) {
+			/* Some are expected. */
+			if (!streq(pkt, "ERROR_PKT:Commit tx noticed")
+			    && !streq(pkt, "ERROR_PKT:Otherspend noticed")
+			    && !streq(pkt, "ERROR_PKT:Anchor timed out")
+			    && !streq(pkt, "ERROR_PKT:Close timed out")) {
+				return pkt;
+			}
+		}
+		assert(sdata->num_outputs < ARRAY_SIZE(sdata->outputs));
+		sdata->outputs[sdata->num_outputs++] = input_by_name(pkt);
+	}
+	if (effect->watch) {
+		/* We can have multiple steals in flight, so make an exception
+		 * for BITCOIN_STEAL_DONE */
+		if (sdata->event_notifies & (1ULL << BITCOIN_STEAL_DONE)
+		    & effect->watch->events)
+			effect->watch->events &= ~(1ULL << BITCOIN_STEAL_DONE);
+
+		if (sdata->event_notifies & effect->watch->events)
+			return "event set twice";
+		sdata->event_notifies |= effect->watch->events;
+		/* Events are not independent. */
+		if (effect->watch->events & BITCOIN_ANCHOR_DEPTHOK)
+			sdata->event_notifies &= ~(1ULL<<BITCOIN_ANCHOR_TIMEOUT);
+		if (effect->watch->events & BITCOIN_ANCHOR_TIMEOUT)
+			sdata->event_notifies &= ~(1ULL<<BITCOIN_ANCHOR_DEPTHOK);
+	}
+	if (effect->unwatch) {
+		if ((sdata->event_notifies & effect->unwatch->events)
+		    != effect->unwatch->events)
+			return "unset event unwatched";
+		sdata->event_notifies &= ~effect->unwatch->events;
+	}
+	if (effect->defer != INPUT_NONE) {
+		/* If it was current command, it is no longer. */
+		if (is_current_command(sdata, effect->defer))
+			sdata->current_command = INPUT_NONE;
+		else if (input_is_pkt(effect->defer)) {
+			/* Unlike commands, which we always resubmit,
+			 * we have to remember deferred packets. */
+			/* We assume only one deferrment! */
+			assert(sdata->deferred_pkt == INPUT_NONE
+			       || sdata->deferred_pkt == effect->defer);
+			sdata->deferred_pkt = effect->defer;
+			sdata->deferred_state = sdata->state;
+		}
+	}
+	if (effect->complete != INPUT_NONE) {
+		if (!is_current_command(sdata, effect->complete))
+			return tal_fmt(NULL, "Completed %s not %s",
+				       input_name(effect->complete),
+				       input_name(sdata->current_command));
+		sdata->current_command = INPUT_NONE;
+	}
+	if (effect->stop_packets) {
+		if (!sdata->pkt_inputs)
+			return "stop_packets twice";
+		sdata->pkt_inputs = false;
+
+		/* Can no longer receive packet timeouts, either. */
+		sdata->event_notifies &= ~(1ULL<<INPUT_CLOSE_COMPLETE_TIMEOUT);
+	}
+	if (effect->stop_commands) {
+		if (!sdata->cmd_inputs)
+			return "stop_commands twice";
+		if (sdata->current_command != INPUT_NONE)
+			return tal_fmt(NULL, "stop_commands with pending command %s",
+				       input_name(sdata->current_command));
+		sdata->cmd_inputs = false;
+	}
+	if (effect->close_timeout != INPUT_NONE) {
+		add_event(&sdata->event_notifies, effect->close_timeout);
+		/* We assume this. */
+		assert(effect->close_timeout == INPUT_CLOSE_COMPLETE_TIMEOUT);
+	}
+	if (effect->in_error) {
+		/* We should stop talking to them after error received. */
+		if (sdata->pkt_inputs)
+			return "packets still open after error pkt";
+	}
+	return NULL;
+}
+	
+static struct trail *try_input(const struct state_data *sdata,
+			       enum state_input i,
+			       struct hist *hist)
+{
+	struct state_data copy, peer;
+	union input idata;
+	struct trail *t;
+	struct state_effect *effect = tal(NULL, struct state_effect);
+	enum state newstate;
+	const char *problem;
+
+	state_effect_init(effect);
+
+	idata.pkt = (Pkt *)tal(effect, char);
+	newstate = state(sdata->state, sdata, i, &idata, effect);
+
+	if (newstate == STATE_ERR_INTERNAL)
+		return new_trail(i, sdata, newstate, effect, "Internal error");
+
+	copy_peers(&copy, &peer, sdata);
+	copy.state = newstate;
+	problem = apply_effects(&copy, effect);
+	if (problem)
+		return new_trail(i, sdata, newstate, effect, problem);
+
+	/* Have we been in this overall situation before? */
+	if (!hist_update(hist, &copy)) {
+		tal_free(effect);
+		return NULL;
+	}
+
+	/* Don't continue if we reached a different error state. */
+	if (state_is_error(newstate)) {
+		tal_free(effect);
+		return NULL;
+	}
+
+	/* Finished? */
+	if (newstate == STATE_CLOSED) {
+		if (copy.pkt_inputs)
+			return new_trail(i, sdata, newstate, effect,
+					 "CLOSED but taking packets?");
+
+		if (copy.cmd_inputs)
+			return new_trail(i, sdata, newstate, effect,
+					 "CLOSED but taking commands?");
+
+		if (copy.current_command != INPUT_NONE)
+			return new_trail(i, sdata, newstate, effect,
+					 input_name(copy.current_command));
+		tal_free(effect);
+		return NULL;
+	}
+
+	/* Try inputs from here down. */
+	t = run_peer(&copy, hist);
+	if (!t)
+		t = run_peer(&peer, hist);
+	if (!t) {
+		tal_free(effect);
+		return NULL;
+	}
+	return add_trail(i, sdata, newstate, effect, t);
+}
+
+static void sanity_check(const struct state_data *sdata)
+{
+	if (sdata->state == STATE_NORMAL_LOWPRIO
+	    || sdata->state == STATE_NORMAL_HIGHPRIO) {
+		/* Home state: expect commands to be finished. */
+		if (sdata->current_command != INPUT_NONE)
+			errx(1, "Unexpected command %u in state %u",
+			     sdata->current_command, sdata->state);
+	}
+}
+
+static void activate_event(struct state_data *sdata, enum state_input i)
+{
+	/* Events are not independent. */
+	switch (i) {
+	case BITCOIN_ANCHOR_DEPTHOK:
+		/* Can't sent TIMEOUT */
+		sdata->event_notifies &= ~(1ULL<<BITCOIN_ANCHOR_TIMEOUT);
+		break;
+	case BITCOIN_ANCHOR_TIMEOUT:
+		/* Can't sent DEPTHOK */
+		sdata->event_notifies &= ~(1ULL<<BITCOIN_ANCHOR_DEPTHOK);
+		break;
+	default:
+		;
+	}
+}
+
+static struct trail *run_peer(const struct state_data *sdata,
+			      struct hist *hist)
+{
+	struct state_data copy, peer;
+	size_t i;
+	uint64_t old_notifies;
+	struct trail *t;
+
+	sanity_check(sdata);
+
+	/* We want to frob some things... */
+	copy_peers(&copy, &peer, sdata);
+	
+	/* Try the event notifiers */
+	old_notifies = copy.event_notifies;
+	for (i = 0; i < 64; i++) {
+		if (!(copy.event_notifies & (1ULL << i)))
+			continue;
+
+		/* Don't re-fire (except OTHERSPEND can reoccur) */
+		if (i != BITCOIN_ANCHOR_OTHERSPEND)
+			copy.event_notifies &= ~(1ULL << i);
+		activate_event(&copy, i);
+		t = try_input(&copy, i, hist);
+		if (t)
+			return t;
+		copy.event_notifies = old_notifies;
+	}
+
+	/* Try sending commands (unless in init state, or closed). */
+	if (sdata->state != STATE_INIT_WITHANCHOR
+	    && sdata->state != STATE_INIT_NOANCHOR
+	    && sdata->cmd_inputs) {
+		/* We don't allow nested commands. */
+		if (sdata->current_command == INPUT_NONE) {
+			size_t i;
+			static const enum state_input cmds[]
+				= { CMD_SEND_UPDATE,
+				    CMD_SEND_HTLC_UPDATE,
+				    CMD_SEND_HTLC_COMPLETE,
+				    CMD_SEND_HTLC_TIMEDOUT,
+				    CMD_SEND_HTLC_ROUTEFAIL,
+				    CMD_CLOSE };
+
+			for (i = 0; i < sizeof(cmds) / sizeof(cmds[i]); i++) {
+				copy.current_command = cmds[i];
+				t = try_input(&copy, cmds[i], hist);
+				if (t)
+					return t;
+			}
+			copy.current_command = INPUT_NONE;
+		}
+	}
+
+	/* Allowed to send inputs? */
+	if (copy.pkt_inputs) {
+		enum state_input i;
+		
+		if (copy.deferred_pkt != INPUT_NONE) {
+			/* Can only resubmit once state changed. */
+			if (copy.state != copy.deferred_state) {
+				i = copy.deferred_pkt;
+				copy.deferred_pkt = INPUT_NONE;
+				return try_input(&copy, i, hist);
+			}
+			/* Can't send anything until that's done. */
+			return NULL;
+		}
+				
+		if (peer.num_outputs) {
+			i = peer.outputs[0];
+
+			/* Do the first, recursion does the rest. */
+			memmove(peer.outputs, peer.outputs + 1,
+				sizeof(peer.outputs) - sizeof(peer.outputs[0]));
+			peer.num_outputs--;
+			return try_input(&copy, i, hist);
+		}
+	}
+	return NULL;
+}
+
+int main(void)
+{
+	struct state_data a, b;
+	unsigned int i;
+	struct hist *hist = tal(NULL, struct hist);
+	struct trail *t;
+
+	/* Initialize universe. */
+	sdata_init(&a, &b, STATE_INIT_WITHANCHOR, "A");
+	sdata_init(&b, &a, STATE_INIT_NOANCHOR, "B");
+	hist_init(hist);
+	if (!hist_update(hist, &a))
+		abort();
+
+	/* Now, try each input in each state. */
+	t = run_peer(&a, hist);
+	if (t) {
+		fprintf(stderr, "Error: %s\n", t->problem);
+		while (t) {
+			fprintf(stderr, "%s: %s %s -> %s\n",
+				t->name,
+				input_name(t->input),
+				state_name(t->before), state_name(t->after));
+			if (t->pkt_sent)
+				fprintf(stderr, "  => %s\n", t->pkt_sent);
+			t = t->next;
+		}
+		exit(1);
+	}
+
+	for (i = 0; i < STATE_MAX; i++) {
+		bool a_expect = true, b_expect = true;
+		/* A supplied anchor, so doesn't enter NOANCHOR states. */
+		if (i == STATE_INIT_NOANCHOR
+		    || i == STATE_OPEN_WAIT_FOR_OPEN_NOANCHOR
+		    || i == STATE_OPEN_WAIT_FOR_ANCHOR
+		    || i == STATE_OPEN_WAITING_THEIRANCHOR
+		    || i == STATE_OPEN_WAIT_FOR_COMPLETE_THEIRANCHOR
+		    || i == STATE_ERR_ANCHOR_TIMEOUT)
+			a_expect = false;
+		if (i == STATE_INIT_WITHANCHOR
+		    || i == STATE_OPEN_WAIT_FOR_OPEN_WITHANCHOR
+		    || i == STATE_OPEN_WAIT_FOR_COMMIT_SIG
+		    || i == STATE_OPEN_WAIT_FOR_COMPLETE_OURANCHOR
+		    || i == STATE_OPEN_WAITING_OURANCHOR)
+			b_expect = false;
+		if (i == STATE_ERR_INTERNAL)
+			a_expect = b_expect = false;
+		if (visited_state(hist, i, 0) != a_expect)
+			warnx("Peer A %s state %s",
+			      a_expect ? "didn't visit" : "visited",
+			      state_name(i));
+		if (visited_state(hist, i, 1) != b_expect)
+			warnx("Peer B %s state %s",
+			     b_expect ? "didn't visit" : "visited",
+			      state_name(i));
+	}
+
+	return 0;
+}