diff --git a/examples/fx_hp1/vst3/Makefile b/examples/fx_hp1/vst3/Makefile
index 3244755..aab2bdd 100644
--- a/examples/fx_hp1/vst3/Makefile
+++ b/examples/fx_hp1/vst3/Makefile
@@ -4,3 +4,6 @@ NAME := bw_example_fx_hp1
 SOURCES = ${SOURCES_COMMON} ${ROOT_DIR}/../src/bw_example_fx_hp1.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_hp1/vst3/Makefile b/examples/fxpp_hp1/vst3/Makefile
index cc744da..d68eb95 100644
--- a/examples/fxpp_hp1/vst3/Makefile
+++ b/examples/fxpp_hp1/vst3/Makefile
@@ -4,3 +4,6 @@ NAME := bw_example_fxpp_hp1
 SOURCES = ${SOURCES_COMMON} ${ROOT_DIR}/../src/bw_example_fxpp_hp1.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_hp1.h b/include/bw_hp1.h
index c97f68f..1da24f2 100644
--- a/include/bw_hp1.h
+++ b/include/bw_hp1.h
@@ -30,6 +30,7 @@
  *    <ul>
  *      <li>Version <strong>1.0.0</strong>:
  *        <ul>
+ *          <li>Added prewarp_at_cutoff and prewarp_freq parameters.</li>
  *          <li><code>bw_hp1_process()</code> and
  *              <code>bw_hp1_process_multi()</code> now use <code>size_t</code>
  *              to count samples and channels.</li>
@@ -39,6 +40,8 @@
  *          <li>Added overladed C++ <code>process()</code> function taking
  *              C-style arrays as arguments.</li>
  *          <li>Removed usage of reserved identifiers.</li>
+ *          <li>Clearly specificed parameter validity ranges.</li>
+ *          <li>Added debugging code.</li>
  *        </ul>
  *      </li>
  *      <li>Version <strong>0.6.0</strong>:
@@ -91,51 +94,68 @@ typedef struct bw_hp1_state bw_hp1_state;
  *
  *    #### bw_hp1_init()
  *  ```>>> */
-static inline void bw_hp1_init(bw_hp1_coeffs *BW_RESTRICT coeffs);
+static inline void bw_hp1_init(
+	bw_hp1_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Initializes input parameter values in `coeffs`.
  *
  *    #### bw_hp1_set_sample_rate()
  *  ```>>> */
-static inline void bw_hp1_set_sample_rate(bw_hp1_coeffs *BW_RESTRICT coeffs, float sample_rate);
+static inline void bw_hp1_set_sample_rate(
+	bw_hp1_coeffs * BW_RESTRICT coeffs,
+	float                       sample_rate);
 /*! <<<```
  *    Sets the `sample_rate` (Hz) value in `coeffs`.
  *
  *    #### bw_hp1_reset_coeffs()
  *  ```>>> */
-static inline void bw_hp1_reset_coeffs(bw_hp1_coeffs *BW_RESTRICT coeffs);
+static inline void bw_hp1_reset_coeffs(
+	bw_hp1_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Resets coefficients in `coeffs` to assume their target values.
  *
  *    #### bw_hp1_reset_state()
  *  ```>>> */
-static inline void bw_hp1_reset_state(const bw_hp1_coeffs *BW_RESTRICT coeffs, bw_hp1_state *BW_RESTRICT state, float x_0);
+static inline void bw_hp1_reset_state(
+	const bw_hp1_coeffs * BW_RESTRICT coeffs,
+	bw_hp1_state * BW_RESTRICT        state,
+	float                             x_0);
 /*! <<<```
  *    Resets the given `state` to its initial values using the given `coeffs`
  *    and the quiescent/initial input value `x_0`.
  *
  *    #### bw_hp1_update_coeffs_ctrl()
  *  ```>>> */
-static inline void bw_hp1_update_coeffs_ctrl(bw_hp1_coeffs *BW_RESTRICT coeffs);
+static inline void bw_hp1_update_coeffs_ctrl(
+	bw_hp1_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Triggers control-rate update of coefficients in `coeffs`.
  *
  *    #### bw_hp1_update_coeffs_audio()
  *  ```>>> */
-static inline void bw_hp1_update_coeffs_audio(bw_hp1_coeffs *BW_RESTRICT coeffs);
+static inline void bw_hp1_update_coeffs_audio(
+	bw_hp1_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Triggers audio-rate update of coefficients in `coeffs`.
  *
  *    #### bw_hp1_process1()
  *  ```>>> */
-static inline float bw_hp1_process1(const bw_hp1_coeffs *BW_RESTRICT coeffs, bw_hp1_state *BW_RESTRICT state, float x);
+static inline float bw_hp1_process1(
+	const bw_hp1_coeffs * BW_RESTRICT coeffs,
+	bw_hp1_state * BW_RESTRICT        state,
+	float                             x);
 /*! <<<```
  *    Processes one input sample `x` using `coeffs`, while using and updating
  *    `state`. Returns the corresponding output sample.
  *
  *    #### bw_hp1_process()
  *  ```>>> */
-static inline void bw_hp1_process(bw_hp1_coeffs *BW_RESTRICT coeffs, bw_hp1_state *BW_RESTRICT state, const float *x, float *y, size_t n_samples);
+static inline void bw_hp1_process(
+	bw_hp1_coeffs * BW_RESTRICT coeffs,
+	bw_hp1_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
@@ -143,7 +163,13 @@ static inline void bw_hp1_process(bw_hp1_coeffs *BW_RESTRICT coeffs, bw_hp1_stat
  *
  *    #### bw_hp1_process_multi()
  *  ```>>> */
-static inline void bw_hp1_process_multi(bw_hp1_coeffs *BW_RESTRICT coeffs, bw_hp1_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_hp1_process_multi(
+	bw_hp1_coeffs * BW_RESTRICT                    coeffs,
+	bw_hp1_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
@@ -152,11 +178,65 @@ static inline void bw_hp1_process_multi(bw_hp1_coeffs *BW_RESTRICT coeffs, bw_hp
  *
  *    #### bw_hp1_set_cutoff()
  *  ```>>> */
-static inline void bw_hp1_set_cutoff(bw_hp1_coeffs *BW_RESTRICT coeffs, float value);
+static inline void bw_hp1_set_cutoff(
+	bw_hp1_coeffs * BW_RESTRICT coeffs,
+	float                       value);
 /*! <<<```
  *    Sets the cutoff frequency `value` (Hz) in `coeffs`.
  *
+ *    Valid range: [`1e-6f`, `1e6f`].
+ *
  *    Default value: `1e3f`.
+ *
+ *    #### bw_hp1_set_prewarp_at_cutoff()
+ *  ```>>> */
+static inline void bw_hp1_set_prewarp_at_cutoff(
+	bw_hp1_coeffs * BW_RESTRICT coeffs,
+	char                        value);
+/*! <<<```
+ *    Sets whether bilinear transform prewarping frequency should match the
+ *    cutoff frequency (non-`0`) or not (`0`).
+ *
+ *    Default value: non-`0` (on).
+ *
+ *    #### bw_hp1_set_prewarp_freq()
+ *  ```>>> */
+static inline void bw_hp1_set_prewarp_freq(
+	bw_hp1_coeffs * BW_RESTRICT coeffs,
+	float                       value);
+/*! <<<```
+ *    Sets the prewarping frequency `value` (Hz) in `coeffs`.
+ *
+ *    Only used when the prewarp\_at\_cutoff parameter is off and however
+ *    internally limited to avoid instability.
+ *
+ *    Valid range: [`1e-6f`, `1e6f`].
+ *
+ *    Default value: `1e3f`.
+ *
+ *    #### bw_hp1_coeffs_is_valid()
+ *  ```>>> */
+static inline char bw_hp1_coeffs_is_valid(
+	const bw_hp1_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_hp1_coeffs`.
+ *
+ *    #### bw_hp1_state_is_valid()
+ *  ```>>> */
+static inline char bw_hp1_state_is_valid(
+	const bw_hp1_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.
+ *
+ *    `state` must at least point to a readable memory block of size greater
+ *    than or equal to that of `bw_hp1_state`.
  *  }}} */
 
 #ifdef __cplusplus
@@ -174,63 +254,273 @@ static inline void bw_hp1_set_cutoff(bw_hp1_coeffs *BW_RESTRICT coeffs, float va
 extern "C" {
 #endif
 
+#ifdef BW_DEBUG_DEEP
+enum bw_hp1_coeffs_state {
+	bw_hp1_coeffs_state_invalid,
+	bw_hp1_coeffs_state_init,
+	bw_hp1_coeffs_state_set_sample_rate,
+	bw_hp1_coeffs_state_reset_coeffs
+};
+#endif
+
 struct bw_hp1_coeffs {
+#ifdef BW_DEBUG_DEEP
+	uint32_t			hash;
+	enum bw_hp1_coeffs_state	state;
+	uint32_t			reset_id;
+#endif
+
 	// Sub-components
 	bw_lp1_coeffs	lp1_coeffs;
 };
 
 struct bw_hp1_state {
+#ifdef BW_DEBUG_DEEP
+	uint32_t	hash;
+	uint32_t	coeffs_reset_id;
+#endif
+
 	bw_lp1_state	lp1_state;
 };
 
-static inline void bw_hp1_init(bw_hp1_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_hp1_init(
+		bw_hp1_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+
 	bw_lp1_init(&coeffs->lp1_coeffs);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->hash = bw_hash_sdbm("bw_hp1_coeffs");
+	coeffs->state = bw_hp1_coeffs_state_init;
+	coeffs->reset_id = coeffs->hash + 1;
+#endif
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_hp1_coeffs_state_init);
 }
 
-static inline void bw_hp1_set_sample_rate(bw_hp1_coeffs *BW_RESTRICT coeffs, float sample_rate) {
+static inline void bw_hp1_set_sample_rate(
+		bw_hp1_coeffs * BW_RESTRICT coeffs,
+		float                       sample_rate) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(sample_rate) && sample_rate > 0.f);
+
 	bw_lp1_set_sample_rate(&coeffs->lp1_coeffs, sample_rate);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->state = bw_hp1_coeffs_state_set_sample_rate;
+#endif
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_hp1_coeffs_state_set_sample_rate);
 }
 
-static inline void bw_hp1_reset_coeffs(bw_hp1_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_hp1_reset_coeffs(
+		bw_hp1_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_set_sample_rate);
+
 	bw_lp1_reset_coeffs(&coeffs->lp1_coeffs);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->state = bw_hp1_coeffs_state_reset_coeffs;
+	coeffs->reset_id++;
+#endif
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_hp1_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_hp1_reset_state(const bw_hp1_coeffs *BW_RESTRICT coeffs, bw_hp1_state *BW_RESTRICT state, float x_0) {
+static inline void bw_hp1_reset_state(
+		const bw_hp1_coeffs * BW_RESTRICT coeffs,
+		bw_hp1_state * BW_RESTRICT        state,
+		float                             x_0) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(bw_is_finite(x_0));
+
 	bw_lp1_reset_state(&coeffs->lp1_coeffs, &state->lp1_state, x_0);
+
+#ifdef BW_DEBUG_DEEP
+	state->hash = bw_hash_sdbm("bw_hp1_state");
+	state->coeffs_reset_id = coeffs->reset_id;
+#endif
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_hp1_state_is_valid(state));
+	BW_ASSERT_DEEP(coeffs->reset_id == state->coeffs_reset_id);
 }
 
-static inline void bw_hp1_update_coeffs_ctrl(bw_hp1_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_hp1_update_coeffs_ctrl(
+		bw_hp1_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_reset_coeffs);
+
 	bw_lp1_update_coeffs_ctrl(&coeffs->lp1_coeffs);
+
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_hp1_update_coeffs_audio(bw_hp1_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_hp1_update_coeffs_audio(
+		bw_hp1_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_reset_coeffs);
+
 	bw_lp1_update_coeffs_audio(&coeffs->lp1_coeffs);
+
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_reset_coeffs);
 }
 
-static inline float bw_hp1_process1(const bw_hp1_coeffs *BW_RESTRICT coeffs, bw_hp1_state *BW_RESTRICT state, float x) {
+static inline float bw_hp1_process1(
+		const bw_hp1_coeffs * BW_RESTRICT coeffs,
+		bw_hp1_state * BW_RESTRICT        state,
+		float                             x) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT_DEEP(bw_hp1_state_is_valid(state));
+	BW_ASSERT_DEEP(coeffs->reset_id == state->coeffs_reset_id);
+	BW_ASSERT(bw_is_finite(x));
+
 	const float lp = bw_lp1_process1(&coeffs->lp1_coeffs, &state->lp1_state, x);
-	return x - lp;
+	const float y = x - lp;
+
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_hp1_state_is_valid(state));
+	BW_ASSERT_DEEP(coeffs->reset_id == state->coeffs_reset_id);
+	BW_ASSERT(bw_is_finite(y));
+
+	return y;
 }
 
-static inline void bw_hp1_process(bw_hp1_coeffs *BW_RESTRICT coeffs, bw_hp1_state *BW_RESTRICT state, const float *x, float *y, size_t n_samples) {
+static inline void bw_hp1_process(
+		bw_hp1_coeffs * BW_RESTRICT coeffs,
+		bw_hp1_state * BW_RESTRICT  state,
+		const float *               x,
+		float *                     y,
+		size_t                      n_samples) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT_DEEP(bw_hp1_state_is_valid(state));
+	BW_ASSERT_DEEP(coeffs->reset_id == state->coeffs_reset_id);
+	BW_ASSERT(x != NULL);
+	BW_ASSERT_DEEP(!bw_has_nan(x, n_samples));
+	BW_ASSERT(y != NULL);
+
 	bw_hp1_update_coeffs_ctrl(coeffs);
 	for (size_t i = 0; i < n_samples; i++) {
 		bw_hp1_update_coeffs_audio(coeffs);
 		y[i] = bw_hp1_process1(coeffs, state, x[i]);
 	}
+
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_hp1_state_is_valid(state));
+	BW_ASSERT_DEEP(coeffs->reset_id == state->coeffs_reset_id);
+	BW_ASSERT_DEEP(!bw_has_nan(y, n_samples));
 }
 
-static inline void bw_hp1_process_multi(bw_hp1_coeffs *BW_RESTRICT coeffs, bw_hp1_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_hp1_process_multi(
+		bw_hp1_coeffs * BW_RESTRICT                    coeffs,
+		bw_hp1_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_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(x != NULL);
+	BW_ASSERT(y != NULL);
+
 	bw_hp1_update_coeffs_ctrl(coeffs);
 	for (size_t i = 0; i < n_samples; i++) {
 		bw_hp1_update_coeffs_audio(coeffs);
 		for (size_t j = 0; j < n_channels; j++)
 			y[j][i] = bw_hp1_process1(coeffs, state[j], x[j][i]);
 	}
+
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_hp1_set_cutoff(bw_hp1_coeffs *BW_RESTRICT coeffs, float value) {
+static inline void bw_hp1_set_cutoff(
+		bw_hp1_coeffs * BW_RESTRICT coeffs,
+		float                       value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_init);
+	BW_ASSERT(!bw_is_nan(value));
+	BW_ASSERT(value >= 1e-6f && value <= 1e6f);
+
 	bw_lp1_set_cutoff(&coeffs->lp1_coeffs, value);
+
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_init);
+}
+
+static inline void bw_hp1_set_prewarp_at_cutoff(
+		bw_hp1_coeffs * BW_RESTRICT coeffs,
+		char                        value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_init);
+
+	bw_lp1_set_prewarp_at_cutoff(&coeffs->lp1_coeffs, value);
+
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_init);
+}
+
+static inline void bw_hp1_set_prewarp_freq(
+		bw_hp1_coeffs * BW_RESTRICT coeffs,
+		float                       value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_init);
+	BW_ASSERT(value >= 1e-6f && value <= 1e6f);
+
+	bw_lp1_set_prewarp_freq(&coeffs->lp1_coeffs, value);
+
+	BW_ASSERT_DEEP(bw_hp1_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_hp1_coeffs_state_init);
+}
+
+static inline char bw_hp1_coeffs_is_valid(
+		const bw_hp1_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+
+#ifdef BW_DEBUG_DEEP
+	if (coeffs->hash != bw_hash_sdbm("bw_hp1_coeffs"))
+		return 0;
+	if (coeffs->state < bw_hp1_coeffs_state_init || coeffs->state > bw_hp1_coeffs_state_reset_coeffs)
+		return 0;
+#endif
+
+	return bw_lp1_coeffs_is_valid(&coeffs->lp1_coeffs);
+}
+
+static inline char bw_hp1_state_is_valid(
+		const bw_hp1_state * BW_RESTRICT state) {
+	BW_ASSERT(state != NULL);
+
+#ifdef BW_DEBUG_DEEP
+	if (state->hash != bw_hash_sdbm("bw_hp1_state"))
+		return 0;
+#endif
+
+	return bw_lp1_state_is_valid(&state->lp1_state);
 }
 
 #ifdef __cplusplus
@@ -250,18 +540,30 @@ class HP1 {
 public:
 	HP1();
 
-	void setSampleRate(float sampleRate);
-	void reset(float x_0 = 0.f);
+	void setSampleRate(
+		float sampleRate);
+
+	void reset(
+		float x0 = 0.f);
+
 	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 setCutoff(float value);
+	void setCutoff(
+		float value);
+
+	void setPrewarpAtCutoff(
+		bool value);
+
+	void setPrewarpFreq(
+		float value);
 /*! <<<...
  *  }
  *  ```
@@ -286,38 +588,53 @@ inline HP1<N_CHANNELS>::HP1() {
 }
 
 template<size_t N_CHANNELS>
-inline void HP1<N_CHANNELS>::setSampleRate(float sampleRate) {
+inline void HP1<N_CHANNELS>::setSampleRate(
+		float sampleRate) {
 	bw_hp1_set_sample_rate(&coeffs, sampleRate);
 }
 
 template<size_t N_CHANNELS>
-inline void HP1<N_CHANNELS>::reset(float x_0) {
+inline void HP1<N_CHANNELS>::reset(
+		float x0) {
 	bw_hp1_reset_coeffs(&coeffs);
 	for (size_t i = 0; i < N_CHANNELS; i++)
-		bw_hp1_reset_state(&coeffs, states + i, x_0);
+		bw_hp1_reset_state(&coeffs, states + i, x0);
 }
 
 template<size_t N_CHANNELS>
 inline void HP1<N_CHANNELS>::process(
-		const float * const *x,
-		float * const *y,
-		size_t nSamples) {
+		const float * const * x,
+		float * const *       y,
+		size_t                nSamples) {
 	bw_hp1_process_multi(&coeffs, statesP, x, y, N_CHANNELS, nSamples);
 }
 
 template<size_t N_CHANNELS>
 inline void HP1<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 HP1<N_CHANNELS>::setCutoff(float value) {
+inline void HP1<N_CHANNELS>::setCutoff(
+		float value) {
 	bw_hp1_set_cutoff(&coeffs, value);
 }
 
+template<size_t N_CHANNELS>
+inline void HP1<N_CHANNELS>::setPrewarpAtCutoff(
+		bool value) {
+	bw_hp1_set_prewarp_at_cutoff(&coeffs, value);
+}
+
+template<size_t N_CHANNELS>
+inline void HP1<N_CHANNELS>::setPrewarpFreq(
+		float value) {
+	bw_hp1_set_prewarp_freq(&coeffs, value);
+}
+
 }
 #endif
 
diff --git a/include/bw_lp1.h b/include/bw_lp1.h
index a5ebc3a..1fedfc8 100644
--- a/include/bw_lp1.h
+++ b/include/bw_lp1.h
@@ -42,6 +42,7 @@
  *              C-style arrays as arguments.</li>
  *          <li>Removed usage of reserved identifiers.</li>
  *          <li>Fixed theoretical bug in <code>bw_lp1_init()</code>.</li>
+ *          <li>Clearly specificed parameter validity ranges.</li>
  *          <li>Added debugging code.</li>
  *        </ul>
  *      </li>
@@ -549,7 +550,7 @@ static inline void bw_lp1_set_prewarp_freq(
 }
 
 static inline char bw_lp1_coeffs_is_valid(
-	const bw_lp1_coeffs * BW_RESTRICT coeffs) {
+		const bw_lp1_coeffs * BW_RESTRICT coeffs) {
 	BW_ASSERT(coeffs != NULL);
 
 #ifdef BW_DEBUG_DEEP
@@ -588,7 +589,7 @@ static inline char bw_lp1_coeffs_is_valid(
 }
 
 static inline char bw_lp1_state_is_valid(
-	const bw_lp1_state * BW_RESTRICT state) {
+		const bw_lp1_state * BW_RESTRICT state) {
 	BW_ASSERT(state != NULL);
 
 #ifdef BW_DEBUG_DEEP
@@ -596,8 +597,7 @@ static inline char bw_lp1_state_is_valid(
 		return 0;
 #endif
 
-	return bw_is_finite(state->y_z1);
-	return bw_is_finite(state->X_z1);
+	return bw_is_finite(state->y_z1) && bw_is_finite(state->X_z1);
 }
 
 #ifdef __cplusplus
diff --git a/include/bw_one_pole.h b/include/bw_one_pole.h
index dcfd2b7..efce7e2 100644
--- a/include/bw_one_pole.h
+++ b/include/bw_one_pole.h
@@ -42,6 +42,7 @@
  *          <li>Removed usage of reserved identifiers.</li>
  *          <li>Now using backward Euler rather than impulse invariant
  *              method.</li>
+ *          <li>Clearly specificed parameter validity ranges.</li>
  *          <li>Added more debugging code.</li>
  *          <li>Added pragmas to silence bogus GCC uninitialized variable
  *              warnings.</li>
diff --git a/include/bw_slew_lim.h b/include/bw_slew_lim.h
index 2548174..bde6cb4 100644
--- a/include/bw_slew_lim.h
+++ b/include/bw_slew_lim.h
@@ -43,6 +43,7 @@
  *              <code>bw_slew_lim_init()</code>.</li>
  *          <li>Fixed documentation of
  *              <code>bw_slew_lim_update_coeffs_audio()</code>.</li>
+ *          <li>Clearly specificed parameter validity ranges.</li>
  *          <li>Added debugging code.</li>
  *        </ul>
  *      </li>
@@ -747,7 +748,7 @@ static inline float bw_slew_lim_get_y_z1(
 }
 
 static inline char bw_slew_lim_coeffs_is_valid(
-	const bw_slew_lim_coeffs * BW_RESTRICT coeffs) {
+		const bw_slew_lim_coeffs * BW_RESTRICT coeffs) {
 	BW_ASSERT(coeffs != NULL);
 
 #ifdef BW_DEBUG_DEEP
@@ -780,7 +781,7 @@ static inline char bw_slew_lim_coeffs_is_valid(
 }
 
 static inline char bw_slew_lim_state_is_valid(
-	const bw_slew_lim_state * BW_RESTRICT state) {
+		const bw_slew_lim_state * BW_RESTRICT state) {
 	BW_ASSERT(state != NULL);
 
 #ifdef BW_DEBUG_DEEP