diff --git a/TODO b/TODO
index 85c718e..3c8457c 100644
--- a/TODO
+++ b/TODO
@@ -77,6 +77,7 @@ code:
 * src inside distortions? w/ control from outside?
 * make all outs nullable?
 * make all reset state outs nullable (svf, phase_gen, ...)?
+* ring mod fx module?
 
 build system:
 * make makefiles handle paths with spaces etc
diff --git a/examples/fx_trem/src/bw_example_fx_trem.c b/examples/fx_trem/src/bw_example_fx_trem.c
index 57a4e89..024806d 100644
--- a/examples/fx_trem/src/bw_example_fx_trem.c
+++ b/examples/fx_trem/src/bw_example_fx_trem.c
@@ -30,7 +30,7 @@ void bw_example_fx_trem_set_sample_rate(bw_example_fx_trem *instance, float samp
 
 void bw_example_fx_trem_reset(bw_example_fx_trem *instance) {
 	bw_trem_reset_coeffs(&instance->trem_coeffs);
-	bw_trem_reset_state(&instance->trem_coeffs, &instance->trem_state);
+	bw_trem_reset_state(&instance->trem_coeffs, &instance->trem_state, 0.f);
 }
 
 void bw_example_fx_trem_process(bw_example_fx_trem *instance, const float** x, float** y, int n_samples) {
diff --git a/examples/fx_trem/vst3/Makefile b/examples/fx_trem/vst3/Makefile
index 4618a88..71a7732 100644
--- a/examples/fx_trem/vst3/Makefile
+++ b/examples/fx_trem/vst3/Makefile
@@ -4,3 +4,6 @@ NAME := bw_example_fx_trem
 SOURCES = ${SOURCES_COMMON} ${ROOT_DIR}/../src/bw_example_fx_trem.c
 
 include ${ROOT_DIR}/../../common/vst3/vst3.mk
+
+CXXFLAGS += -DRELEASE=1 -DNDEBUG -DBW_NO_DEBUG
+#CXXFLAGS += -DDEVELOPMENT=1 -DBW_DEBUG_DEEP
diff --git a/examples/fxpp_trem/vst3/Makefile b/examples/fxpp_trem/vst3/Makefile
index dce6d58..d9368ae 100644
--- a/examples/fxpp_trem/vst3/Makefile
+++ b/examples/fxpp_trem/vst3/Makefile
@@ -4,3 +4,6 @@ NAME := bw_example_fxpp_trem
 SOURCES = ${SOURCES_COMMON} ${ROOT_DIR}/../src/bw_example_fxpp_trem.cpp
 
 include ${ROOT_DIR}/../../common/vst3/vst3.mk
+
+CXXFLAGS += -DRELEASE=1 -DNDEBUG -DBW_NO_DEBUG
+#CXXFLAGS += -DDEVELOPMENT=1 -DBW_DEBUG_DEEP
diff --git a/include/bw_trem.h b/include/bw_trem.h
index 556252c..667be68 100644
--- a/include/bw_trem.h
+++ b/include/bw_trem.h
@@ -31,6 +31,14 @@
  *    <ul>
  *      <li>Version <strong>1.0.0</strong>:
  *        <ul>
+ *          <li>Added initial input value to
+ *              <code>bw_trem_reset_state()</code>.</li>
+ *          <li>Added <code>bw_trem_reset_state_multi()</code> and updated C++
+ *              API in this regard.</li>
+ *          <li>Now <code>bw_trem_reset_state()</code> returns the initial
+ *              output value.</li>
+ *          <li>Added overloaded C++ <code>reset()</code> functions taking
+ *              arrays as arguments.</li>
  *          <li><code>bw_trem_process()</code> and
  *              <code>bw_trem_process_multi()</code> now use <code>size_t</code>
  *              to count samples and channels.</li>
@@ -40,6 +48,8 @@
  *          <li>Added overloaded C++ <code>process()</code> function taking
  *              C-style arrays as arguments.</li>
  *          <li>Removed usage of reserved identifiers.</li>
+ *          <li>Clearly specified parameter validity ranges.</li>
+ *          <li>Added debugging code.</li>
  *        </ul>
  *      </li>
  *      <li>Version <strong>0.6.0</strong>:
@@ -86,50 +96,86 @@ typedef struct bw_trem_state bw_trem_state;
  *
  *    #### bw_trem_init()
  *  ```>>> */
-static inline void bw_trem_init(bw_trem_coeffs *BW_RESTRICT coeffs);
+static inline void bw_trem_init(
+	bw_trem_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Initializes input parameter values in `coeffs`.
  *
  *    #### bw_trem_set_sample_rate()
  *  ```>>> */
-static inline void bw_trem_set_sample_rate(bw_trem_coeffs *BW_RESTRICT coeffs, float sample_rate);
+static inline void bw_trem_set_sample_rate(
+	bw_trem_coeffs * BW_RESTRICT coeffs,
+	float                        sample_rate);
 /*! <<<```
  *    Sets the `sample_rate` (Hz) value in `coeffs`.
  *
  *    #### bw_trem_reset_coeffs()
  *  ```>>> */
-static inline void bw_trem_reset_coeffs(bw_trem_coeffs *BW_RESTRICT coeffs);
+static inline void bw_trem_reset_coeffs(
+	bw_trem_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Resets coefficients in `coeffs` to assume their target values.
  *
  *    #### bw_trem_reset_state()
  *  ```>>> */
-static inline void bw_trem_reset_state(const bw_trem_coeffs *BW_RESTRICT coeffs, bw_trem_state *BW_RESTRICT state);
+static inline float bw_trem_reset_state(
+	const bw_trem_coeffs * BW_RESTRICT coeffs,
+	bw_trem_state * BW_RESTRICT        state,
+	float                              x_0);
 /*! <<<```
- *    Resets the given `state` to its initial values using the given `coeffs`.
+ *    Resets the given `state` to its initial values using the given `coeffs`
+ *    and the initial input value `x_0`.
+ *
+ *    Returns the corresponding initial output value.
+ *
+ *    #### bw_trem_reset_state_multi()
+ *  ```>>> */
+static inline void bw_trem_reset_state_multi(
+	const bw_trem_coeffs * BW_RESTRICT              coeffs,
+	bw_trem_state * BW_RESTRICT const * BW_RESTRICT state,
+	const float *                                   x_0,
+	float *                                         y_0,
+	size_t                                          n_channels);
+/*! <<<```
+ *    Resets each of the `n_channels` `state`s to its initial values using the
+ *    given `coeffs` and the corresponding initial input value in the `x_0`
+ *    array.
+ *
+ *    The corresponding initial output values are written into the `y_0` array,
+ *    if not `NULL`.
  *
  *    #### bw_trem_update_coeffs_ctrl()
  *  ```>>> */
-static inline void bw_trem_update_coeffs_ctrl(bw_trem_coeffs *BW_RESTRICT coeffs);
+static inline void bw_trem_update_coeffs_ctrl(
+	bw_trem_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Triggers control-rate update of coefficients in `coeffs`.
  *
  *    #### bw_trem_update_coeffs_audio()
  *  ```>>> */
-static inline void bw_trem_update_coeffs_audio(bw_trem_coeffs *BW_RESTRICT coeffs);
+static inline void bw_trem_update_coeffs_audio(
+	bw_trem_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Triggers audio-rate update of coefficients in `coeffs`.
  *
  *    #### bw_trem_process1()
  *  ```>>> */
-static inline float bw_trem_process1(const bw_trem_coeffs *BW_RESTRICT coeffs, bw_trem_state *BW_RESTRICT state, float x);
+static inline float bw_trem_process1(
+	const bw_trem_coeffs * BW_RESTRICT coeffs,
+	bw_trem_state * BW_RESTRICT        state,
+	float                              x);
 /*! <<<```
  *    Processes one input sample `x` using `coeffs`, while using and updating
  *    `state`. Returns the corresponding output sample.
  *
  *    #### bw_trem_process()
  *  ```>>> */
-static inline void bw_trem_process(bw_trem_coeffs *BW_RESTRICT coeffs, bw_trem_state *BW_RESTRICT state, const float *x, float *y, size_t n_samples);
+static inline void bw_trem_process(
+	bw_trem_coeffs * BW_RESTRICT coeffs,
+	bw_trem_state * BW_RESTRICT  state,
+	const float *                x,
+	float *                      y,
+	size_t                       n_samples);
 /*! <<<```
  *    Processes the first `n_samples` of the input buffer `x` and fills the
  *    first `n_samples` of the output buffer `y`, while using and updating both
@@ -137,7 +183,13 @@ static inline void bw_trem_process(bw_trem_coeffs *BW_RESTRICT coeffs, bw_trem_s
  *
  *    #### bw_trem_process_multi()
  *  ```>>> */
-static inline void bw_trem_process_multi(bw_trem_coeffs *BW_RESTRICT coeffs, bw_trem_state *BW_RESTRICT const *BW_RESTRICT state, const float * const *x, float * const *y, size_t n_channels, size_t n_samples);
+static inline void bw_trem_process_multi(
+	bw_trem_coeffs * BW_RESTRICT                    coeffs,
+	bw_trem_state * BW_RESTRICT const * BW_RESTRICT state,
+	const float * const *                           x,
+	float * const *                                 y,
+	size_t                                          n_channels,
+	size_t                                          n_samples);
 /*! <<<```
  *    Processes the first `n_samples` of the `n_channels` input buffers `x` and
  *    fills the first `n_samples` of the `n_channels` output buffers `y`, while
@@ -146,20 +198,57 @@ static inline void bw_trem_process_multi(bw_trem_coeffs *BW_RESTRICT coeffs, bw_
  *
  *    #### bw_trem_set_rate()
  *  ```>>> */
-static inline void bw_trem_set_rate(bw_trem_coeffs *BW_RESTRICT coeffs, float value);
+static inline void bw_trem_set_rate(
+	bw_trem_coeffs * BW_RESTRICT coeffs,
+	float                        value);
 /*! <<<```
  *    Sets the modulation rate `value` (Hz) in `coeffs`.
  *
+ *    `value` must be finite.
+ *
  *    Default value: `1.f`.
  *
  *    #### bw_trem_set_amount()
  *  ```>>> */
-static inline void bw_trem_set_amount(bw_trem_coeffs *BW_RESTRICT coeffs, float value);
+static inline void bw_trem_set_amount(
+	bw_trem_coeffs * BW_RESTRICT coeffs,
+	float                        value);
 /*! <<<```
  *    Sets the amount parameter to the given `value` (`0.f` = no tremolo, `1.f`
  *    = full tremolo, `-1.f` = full tremolo with inverted polarity) in `coeffs`.
  *
+ *    Valid range: [`-1.f` (full tremolo with inverted polarity),
+ *    `1.f` (full tremolo)].
+ *
  *    Default value: `1.f`.
+ *
+*    #### bw_trem_coeffs_is_valid()
+ *  ```>>> */
+static inline char bw_trem_coeffs_is_valid(
+	const bw_trem_coeffs * BW_RESTRICT coeffs);
+/*! <<<```
+ *    Tries to determine whether `coeffs` is valid and returns non-`0` if it
+ *    seems to be the case and `0` if it is certainly not. False positives are
+ *    possible, false negatives are not.
+ *
+ *    `coeffs` must at least point to a readable memory block of size greater
+ *    than or equal to that of `bw_trem_coeffs`.
+ *
+ *    #### bw_trem_state_is_valid()
+ *  ```>>> */
+static inline char bw_trem_state_is_valid(
+	const bw_trem_coeffs * BW_RESTRICT coeffs,
+	const bw_trem_state * BW_RESTRICT  state);
+/*! <<<```
+ *    Tries to determine whether `state` is valid and returns non-`0` if it
+ *    seems to be the case and `0` if it is certainly not. False positives are
+ *    possible, false negatives are not.
+ *
+ *    If `coeffs` is not `NULL` extra cross-checks might be performed (`state`
+ *    is supposed to be associated to `coeffs`).
+ *
+ *    `state` must at least point to a readable memory block of size greater
+ *    than or equal to that of `bw_trem_state`.
  *  }}} */
 
 #ifdef __cplusplus
@@ -179,75 +268,298 @@ static inline void bw_trem_set_amount(bw_trem_coeffs *BW_RESTRICT coeffs, float
 extern "C" {
 #endif
 
+#ifdef BW_DEBUG_DEEP
+enum bw_trem_coeffs_state {
+	bw_trem_coeffs_state_invalid,
+	bw_trem_coeffs_state_init,
+	bw_trem_coeffs_state_set_sample_rate,
+	bw_trem_coeffs_state_reset_coeffs
+};
+#endif
+
 struct bw_trem_coeffs {
+#ifdef BW_DEBUG_DEEP
+	uint32_t			hash;
+	enum bw_trem_coeffs_state	state;
+	uint32_t			reset_id;
+#endif
+
 	// Sub-components
-	bw_phase_gen_coeffs	phase_gen_coeffs;
-	bw_ring_mod_coeffs	ring_mod_coeffs;
+	bw_phase_gen_coeffs		phase_gen_coeffs;
+	bw_ring_mod_coeffs		ring_mod_coeffs;
 };
 
 struct bw_trem_state {
+#ifdef BW_DEBUG_DEEP
+	uint32_t	hash;
+	uint32_t	coeffs_reset_id;
+#endif
+
+	// Sub-components
 	bw_phase_gen_state	phase_gen_state;
 };
 
-static inline void bw_trem_init(bw_trem_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_trem_init(
+		bw_trem_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+
 	bw_phase_gen_init(&coeffs->phase_gen_coeffs);
 	bw_ring_mod_init(&coeffs->ring_mod_coeffs);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->hash = bw_hash_sdbm("bw_trem_coeffs");
+	coeffs->state = bw_trem_coeffs_state_init;
+	coeffs->reset_id = coeffs->hash + 1;
+#endif
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_trem_coeffs_state_init);
 }
 
-static inline void bw_trem_set_sample_rate(bw_trem_coeffs *BW_RESTRICT coeffs, float sample_rate) {
+static inline void bw_trem_set_sample_rate(
+		bw_trem_coeffs * BW_RESTRICT coeffs,
+		float                        sample_rate) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(sample_rate) && sample_rate > 0.f);
+
 	bw_phase_gen_set_sample_rate(&coeffs->phase_gen_coeffs, sample_rate);
 	bw_ring_mod_set_sample_rate(&coeffs->ring_mod_coeffs, sample_rate);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->state = bw_trem_coeffs_state_set_sample_rate;
+#endif
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_trem_coeffs_state_set_sample_rate);
 }
 
-static inline void bw_trem_reset_coeffs(bw_trem_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_trem_reset_coeffs(
+		bw_trem_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_set_sample_rate);
+
 	bw_phase_gen_reset_coeffs(&coeffs->phase_gen_coeffs);
 	bw_ring_mod_reset_coeffs(&coeffs->ring_mod_coeffs);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->state = bw_trem_coeffs_state_reset_coeffs;
+	coeffs->reset_id++;
+#endif
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_trem_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_trem_reset_state(const bw_trem_coeffs *BW_RESTRICT coeffs, bw_trem_state *BW_RESTRICT state) {
-	bw_phase_gen_reset_state(&coeffs->phase_gen_coeffs, &state->phase_gen_state, 0.f);
+static inline float bw_trem_reset_state(
+		const bw_trem_coeffs * BW_RESTRICT coeffs,
+		bw_trem_state * BW_RESTRICT        state,
+		float                              x_0) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(bw_is_finite(x_0));
+
+	float p, pi;
+	bw_phase_gen_reset_state(&coeffs->phase_gen_coeffs, &state->phase_gen_state, 0.f, &p, &pi);
+	const float c = bw_osc_sin_process1(p);
+	const float y = bw_ring_mod_process1(&coeffs->ring_mod_coeffs, x_0, 1.f + c);
+
+#ifdef BW_DEBUG_DEEP
+	state->hash = bw_hash_sdbm("bw_trem_state");
+	state->coeffs_reset_id = coeffs->reset_id;
+#endif
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_trem_state_is_valid(coeffs, state));
+	BW_ASSERT(bw_is_finite(y));
+
+	return y;
 }
 
-static inline void bw_trem_update_coeffs_ctrl(bw_trem_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_trem_reset_state_multi(
+		const bw_trem_coeffs * BW_RESTRICT              coeffs,
+		bw_trem_state * BW_RESTRICT const * BW_RESTRICT state,
+		const float *                                   x_0,
+		float *                                         y_0,
+		size_t                                          n_channels) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(x_0 != NULL);
+
+	if (y_0 != NULL)
+		for (size_t i = 0; i < n_channels; i++)
+			y_0[i] = bw_trem_reset_state(coeffs, state[i], x_0[i]);
+	else
+		for (size_t i = 0; i < n_channels; i++)
+			bw_trem_reset_state(coeffs, state[i], x_0[i]);
+
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(y_0 != NULL ? bw_has_only_finite(y_0, n_channels) : 1);
+}
+
+static inline void bw_trem_update_coeffs_ctrl(
+		bw_trem_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
+
 	bw_phase_gen_update_coeffs_ctrl(&coeffs->phase_gen_coeffs);
 	bw_ring_mod_update_coeffs_ctrl(&coeffs->ring_mod_coeffs);
+
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_trem_update_coeffs_audio(bw_trem_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_trem_update_coeffs_audio(
+		bw_trem_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
+
 	bw_phase_gen_update_coeffs_audio(&coeffs->phase_gen_coeffs);
 	bw_ring_mod_update_coeffs_audio(&coeffs->ring_mod_coeffs);
+
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
 }
 
-static inline float bw_trem_process1(const bw_trem_coeffs *BW_RESTRICT coeffs, bw_trem_state *BW_RESTRICT state, float x) {
+static inline float bw_trem_process1(
+		const bw_trem_coeffs * BW_RESTRICT coeffs,
+		bw_trem_state * BW_RESTRICT        state,
+		float                              x) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT_DEEP(bw_trem_state_is_valid(coeffs, state));
+	BW_ASSERT(bw_is_finite(x));
+
 	float p, pi;
 	bw_phase_gen_process1(&coeffs->phase_gen_coeffs, &state->phase_gen_state, &p, &pi);
 	const float c = bw_osc_sin_process1(p);
-	return bw_ring_mod_process1(&coeffs->ring_mod_coeffs, x, 1.f + c);
+	const float y = bw_ring_mod_process1(&coeffs->ring_mod_coeffs, x, 1.f + c);
+
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_trem_state_is_valid(coeffs, state));
+	BW_ASSERT(bw_is_finite(y));
+
+	return y;
 }
 
-static inline void bw_trem_process(bw_trem_coeffs *BW_RESTRICT coeffs, bw_trem_state *BW_RESTRICT state, const float *x, float *y, size_t n_samples) {
+static inline void bw_trem_process(
+		bw_trem_coeffs * BW_RESTRICT coeffs,
+		bw_trem_state * BW_RESTRICT  state,
+		const float *                x,
+		float *                      y,
+		size_t                       n_samples) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT_DEEP(bw_trem_state_is_valid(coeffs, state));
+	BW_ASSERT(x != NULL);
+	BW_ASSERT_DEEP(bw_has_only_finite(x, n_samples));
+	BW_ASSERT(y != NULL);
+
 	bw_trem_update_coeffs_ctrl(coeffs);
 	for (size_t i = 0; i < n_samples; i++) {
 		bw_trem_update_coeffs_audio(coeffs);
 		y[i] = bw_trem_process1(coeffs, state, x[i]);
 	}
+
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_trem_state_is_valid(coeffs, state));
+	BW_ASSERT_DEEP(bw_has_only_finite(y, n_samples));
 }
 
-static inline void bw_trem_process_multi(bw_trem_coeffs *BW_RESTRICT coeffs, bw_trem_state *BW_RESTRICT const *BW_RESTRICT state, const float * const *x, float * const *y, size_t n_channels, size_t n_samples) {
+static inline void bw_trem_process_multi(
+		bw_trem_coeffs * BW_RESTRICT                    coeffs,
+		bw_trem_state * BW_RESTRICT const * BW_RESTRICT state,
+		const float * const *                           x,
+		float * const *                                 y,
+		size_t                                          n_channels,
+		size_t                                          n_samples) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(x != NULL);
+	BW_ASSERT(y != NULL);
+
 	bw_trem_update_coeffs_ctrl(coeffs);
 	for (size_t i = 0; i < n_samples; i++) {
 		bw_trem_update_coeffs_audio(coeffs);
 		for (size_t j = 0; j < n_channels; j++)
 			y[j][i] = bw_trem_process1(coeffs, state[j], x[j][i]);
 	}
+
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_trem_set_rate(bw_trem_coeffs *BW_RESTRICT coeffs, float value) {
+static inline void bw_trem_set_rate(
+		bw_trem_coeffs * BW_RESTRICT coeffs,
+		float                        value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(value));
+
 	bw_phase_gen_set_frequency(&coeffs->phase_gen_coeffs, value);
+
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_init);
 }
 
-static inline void bw_trem_set_amount(bw_trem_coeffs *BW_RESTRICT coeffs, float value) {
+static inline void bw_trem_set_amount(
+		bw_trem_coeffs * BW_RESTRICT coeffs,
+		float                        value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(value));
+	BW_ASSERT(value >= -1.f && value <= 1.f);
+
 	bw_ring_mod_set_amount(&coeffs->ring_mod_coeffs, value);
+
+	BW_ASSERT_DEEP(bw_trem_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_trem_coeffs_state_init);
+}
+
+static inline char bw_trem_coeffs_is_valid(
+		const bw_trem_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+
+#ifdef BW_DEBUG_DEEP
+	if (coeffs->hash != bw_hash_sdbm("bw_trem_coeffs"))
+		return 0;
+	if (coeffs->state < bw_trem_coeffs_state_init || coeffs->state > bw_trem_coeffs_state_reset_coeffs)
+		return 0;
+#endif
+
+	return bw_phase_gen_coeffs_is_valid(&coeffs->phase_gen_coeffs) && bw_ring_mod_coeffs_is_valid(&coeffs->ring_mod_coeffs);
+}
+
+static inline char bw_trem_state_is_valid(
+		const bw_trem_coeffs * BW_RESTRICT coeffs,
+		const bw_trem_state * BW_RESTRICT  state) {
+	BW_ASSERT(state != NULL);
+
+#ifdef BW_DEBUG_DEEP
+	if (state->hash != bw_hash_sdbm("bw_trem_state"))
+		return 0;
+
+	if (coeffs != NULL && coeffs->reset_id != state->coeffs_reset_id)
+		return 0;
+#endif
+
+	return bw_phase_gen_state_is_valid(coeffs ? &coeffs->phase_gen_coeffs : NULL, &state->phase_gen_state);
 }
 
 #ifdef __cplusplus
@@ -267,19 +579,40 @@ class Trem {
 public:
 	Trem();
 
-	void setSampleRate(float sampleRate);
-	void reset();
+	void setSampleRate(
+		float sampleRate);
+
+	void reset(
+		float               x0 = 0.f,
+		float * BW_RESTRICT y0 = nullptr);
+
+	void reset(
+		float                                       x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0);
+
+	void reset(
+		const float * x0,
+		float *       y0 = nullptr);
+
+	void reset(
+		std::array<float, N_CHANNELS>               x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0 = nullptr);
+
 	void process(
-		const float * const *x,
-		float * const *y,
-		size_t nSamples);
+		const float * const * x,
+		float * const *       y,
+		size_t                nSamples);
+
 	void process(
 		std::array<const float *, N_CHANNELS> x,
-		std::array<float *, N_CHANNELS> y,
-		size_t nSamples);
+		std::array<float *, N_CHANNELS>       y,
+		size_t                                nSamples);
 
-	void setRate(float value);
-	void setAmount(float value);
+	void setRate(
+		float value);
+
+	void setAmount(
+		float value);
 /*! <<<...
  *  }
  *  ```
@@ -291,9 +624,9 @@ public:
  * change at any time in future versions. Please, do not use it directly. */
 
 private:
-	bw_trem_coeffs	 coeffs;
-	bw_trem_state	 states[N_CHANNELS];
-	bw_trem_state	*BW_RESTRICT statesP[N_CHANNELS];
+	bw_trem_coeffs			coeffs;
+	bw_trem_state			states[N_CHANNELS];
+	bw_trem_state * BW_RESTRICT	statesP[N_CHANNELS];
 };
 
 template<size_t N_CHANNELS>
@@ -304,40 +637,71 @@ inline Trem<N_CHANNELS>::Trem() {
 }
 
 template<size_t N_CHANNELS>
-inline void Trem<N_CHANNELS>::setSampleRate(float sampleRate) {
+inline void Trem<N_CHANNELS>::setSampleRate(
+		float sampleRate) {
 	bw_trem_set_sample_rate(&coeffs, sampleRate);
 }
 
 template<size_t N_CHANNELS>
-inline void Trem<N_CHANNELS>::reset() {
+inline void Trem<N_CHANNELS>::reset(
+		float               x0,
+		float * BW_RESTRICT y0) {
 	bw_trem_reset_coeffs(&coeffs);
-	for (size_t i = 0; i < N_CHANNELS; i++)
-		bw_trem_reset_state(&coeffs, states + i);
+	if (y0 != nullptr)
+		for (size_t i = 0; i < N_CHANNELS; i++)
+			y0[i] = bw_trem_reset_state(&coeffs, states + i, x0);
+	else
+		for (size_t i = 0; i < N_CHANNELS; i++)
+			bw_trem_reset_state(&coeffs, states + i, x0);
+}
+
+template<size_t N_CHANNELS>
+inline void Trem<N_CHANNELS>::reset(
+		float                                       x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0) {
+	reset(x0, y0 != nullptr ? y0->data() : nullptr);
+}
+
+template<size_t N_CHANNELS>
+inline void Trem<N_CHANNELS>::reset(
+		const float * x0,
+		float *       y0) {
+	bw_trem_reset_coeffs(&coeffs);
+	bw_trem_reset_state_multi(&coeffs, statesP, x0, y0, N_CHANNELS);
+}
+
+template<size_t N_CHANNELS>
+inline void Trem<N_CHANNELS>::reset(
+		std::array<float, N_CHANNELS>               x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0) {
+	reset(x0.data(), y0 != nullptr ? y0->data() : nullptr);
 }
 
 template<size_t N_CHANNELS>
 inline void Trem<N_CHANNELS>::process(
-		const float * const *x,
-		float * const *y,
-		size_t nSamples) {
+		const float * const * x,
+		float * const *       y,
+		size_t                nSamples) {
 	bw_trem_process_multi(&coeffs, statesP, x, y, N_CHANNELS, nSamples);
 }
 
 template<size_t N_CHANNELS>
 inline void Trem<N_CHANNELS>::process(
 		std::array<const float *, N_CHANNELS> x,
-		std::array<float *, N_CHANNELS> y,
-		size_t nSamples) {
+		std::array<float *, N_CHANNELS>       y,
+		size_t                                nSamples) {
 	process(x.data(), y.data(), nSamples);
 }
 
 template<size_t N_CHANNELS>
-inline void Trem<N_CHANNELS>::setRate(float value) {
+inline void Trem<N_CHANNELS>::setRate(
+		float value) {
 	bw_trem_set_rate(&coeffs, value);
 }
 
 template<size_t N_CHANNELS>
-inline void Trem<N_CHANNELS>::setAmount(float value) {
+inline void Trem<N_CHANNELS>::setAmount(
+		float value) {
 	bw_trem_set_amount(&coeffs, value);
 }