diff --git a/include/spho/spho.h b/include/spho/spho.h
index c3daaa0..e22d7c6 100644
--- a/include/spho/spho.h
+++ b/include/spho/spho.h
@@ -27,20 +27,20 @@ do {									\
 
 #define SPHO_STRINGIFY(a)	#a
 #define SPHO_MACRO_STR(b)	SPHO_STRINGIFY(b)
+#define __LINE__S		SPHO_MACRO_STR(__LINE__)
 
-#define SPHO_RIP(msg) \
+
+#define SPHO_RIP(msg, ...) \
 	do { \
-		fprintf(stderr, "SPHO_RIP(" msg ")@"	\
-		    __FILE__ ":" __LINE__S			\
-		    " failed. Aborting.\n");			\
+		fprintf(stderr, "SPHO_RIP(" msg ")@" \
+		    __FILE__ ":" __LINE__S \
+		    " failed. Aborting.\n" __VA_OPT__(,) __VA_ARGS__); \
 		abort(); \
 	} while (0)
 
 
 #ifdef SPHO_DEBUG
 
-#define __LINE__S		SPHO_MACRO_STR(__LINE__)
-
 #define SPHO_PRECOND(cond) \
 	do {								\
 		if (! (cond) ) {					\
diff --git a/include/spho/subt.h b/include/spho/subt.h
index c185107..f5a0c22 100644
--- a/include/spho/subt.h
+++ b/include/spho/subt.h
@@ -1,6 +1,9 @@
 #ifndef _SPHO_SUBT_H
 #define _SPHO_SUBT_H
 
-#include "spho/bind.h"
+#include "spho/scope.h"
+
+int spho_is_subt(struct spho_ctx *, const struct spho_tp *,
+    const struct spho_tp *);
 
 #endif /* ifndef _SPHO_SUBT_H */
diff --git a/src/spho/subt.c b/src/spho/subt.c
index 062e021..ec2a8fe 100644
--- a/src/spho/subt.c
+++ b/src/spho/subt.c
@@ -9,36 +9,39 @@
 
 #include "spho/subt.h"
 
-struct bound_tp {
+/* bound type */
+struct bound {
 	struct spho_bind		*bind;
 	const struct spho_tp		*tp;
 };
 
-struct seqnt {
+/* sequent */
+struct sqnt {
 	struct spho_ctx		*ctx;
 
 	size_t			sz;
 	size_t			filled;
-	struct bound_tp		*elms;
+	struct bound		*elms;
 };
 
-struct seqnt_tstl {
-	struct seqnt		left;
-	struct seqnt		right;
+/* sequent turnstile */
+struct tstl {
+	struct sqnt		left;
+	struct sqnt		right;
 };
 
 
-static struct bound_tp *spho_bound_tp(const struct spho_tp *);
+static struct bound *bound_tp(const struct spho_tp *);
 
 
 /* read scopes and create bound type */
-struct bound_tp *
-spho_bound_tp(const struct spho_tp *tp)
+struct bound *
+bound_tp(const struct spho_tp *tp)
 {
 	struct spho_scope	*sc;
 	struct spho_bind	*bind, *more_bind;
 
-	struct bound_tp	*ret;
+	struct bound	*ret;
 
 	SPHO_PRECOND(tp != NULL);
 	SPHO_PRECOND(tp->sc != NULL);
@@ -95,17 +98,12 @@ err:
 	return (NULL);
 }
 
-static struct seqnt *
-seqnt_alloc(struct spho_ctx *ctx, size_t initsz)
+static int
+sqnt_init(struct spho_ctx *ctx, size_t initsz, struct sqnt *seq)
 {
-	struct seqnt *seq;
 	if (initsz == 0)
 		initsz = 32; /* just feels good */
 
-	if ((seq = calloc(1, sizeof(*seq))) == NULL) {
-		SPHO_ERR(ctx, SPHO_ERR_SYS);
-		return (NULL);
-	}
 	seq->ctx = ctx;
 
 	if ((seq->elms = calloc(initsz, sizeof(*seq->elms))) == NULL) {
@@ -116,22 +114,20 @@ seqnt_alloc(struct spho_ctx *ctx, size_t initsz)
 	seq->sz = initsz;
 	seq->filled = 0;
 
-	return (seq);
+	return (0);
 err:
-	if (seq != NULL) {
-		if (seq->elms != NULL)
-			free(seq->elms);
-		free(seq);
-	}
+	if (seq->elms != NULL)
+		free(seq->elms);
+	seq->elms = NULL;
 
-	return (NULL);
+	return (-1);
 }
 
 static int
-seqnt_makebig(struct seqnt *seq)
+sqnt_makebig(struct sqnt *seq)
 {
 	size_t		new_sz;
-	struct bound_tp *new_elms;
+	struct bound *new_elms;
 
 	SPHO_PRECOND(seq != NULL);
 	SPHO_PRECOND(seq->sz > 0);
@@ -152,11 +148,11 @@ seqnt_makebig(struct seqnt *seq)
 }
 
 static int
-seqnt_insert_from_ptr(struct seqnt *seq, struct bound_tp *elm)
+sqnt_add_bound(struct sqnt *seq, struct bound *elm)
 {
 	SPHO_PRECOND(seq->filled <= seq->sz);
 
-	if (seq->filled >= seq->sz && (seqnt_makebig(seq) == -1))
+	if (seq->filled >= seq->sz && (sqnt_makebig(seq) == -1))
 		return (-1);
 
 	seq->elms[seq->filled++] = *elm;
@@ -167,38 +163,76 @@ seqnt_insert_from_ptr(struct seqnt *seq, struct bound_tp *elm)
 }
 
 static int
-seqnt_insert_bind_tp(struct seqnt *seq, struct spho_bind *elm_bind,
-    struct spho_tp *elm_tp)
+tstl_init(struct spho_ctx *ctx, size_t lsz, size_t rsz,
+    struct tstl *ts)
 {
-	SPHO_PRECOND(seq->filled <= seq->sz);
-
-	if (seq->filled >= seq->sz && (seqnt_makebig(seq) == -1))
+	if (sqnt_init(ctx, lsz, &ts->left) == -1)
+		return (-1);
+	if (sqnt_init(ctx, rsz, &ts->right) == -1)
 		return (-1);
-
-	seq->elms[seq->filled++] = (struct bound_tp){ elm_bind, elm_tp };
-
-	SPHO_POSTCOND(seq->filled <= seq->sz);
 
 	return (0);
 }
 
+static struct tstl *
+tstl_alloc(struct spho_ctx *ctx)
+{
+	struct tstl *ts;
+
+	if ((ts = calloc(1, sizeof(*ts))) == NULL) {
+		SPHO_ERR(ctx, SPHO_ERR_SYS);
+		return (NULL);
+	}
+	if (tstl_init(ctx, 0, 0, ts) == -1) {
+		free(ts);
+		return (NULL);
+	}
+
+	return (ts);
+}
+
+static int
+tstl_add_left(struct tstl *ts, struct bound *tp)
+{
+	return (sqnt_add_bound(&ts->left, tp));
+}
+
+static int
+tstl_add_right(struct tstl *ts, struct bound *tp)
+{
+	return (sqnt_add_bound(&ts->right, tp));
+}
+
+static int
+tstl_decide(struct tstl *ts)
+{
+	SPHO_RIP("implement");
+}
 
 int
-spho_is_subt(struct spho_tp *tp_left, struct spho_tp *tp_right)
+spho_is_subt(struct spho_ctx *ctx, const struct spho_tp *tp_left,
+    const struct spho_tp *tp_right)
 {
-	struct bound_tp *left, *right;
+	struct tstl ts;
+	struct bound *left, *right;
 
-	if ((left = spho_bound_tp(tp_left)) == NULL)
+	if ((left = bound_tp(tp_left)) == NULL)
 		return (-1);
-	if ((right = spho_bound_tp(tp_right)) == NULL)
+	if ((right = bound_tp(tp_right)) == NULL)
 		return (-1);
 
-	// TODO check well-definedness (e.g. no undefs)
+	// TODO check well-definedness of types (e.g. no undefs)
 
-	// XXX skip boxing step
-	
-	// TODO create seqnt and seqnt_tstl
-	// TODO run decider
+	if ((tstl_init(ctx, 0, 0, &ts)) == -1)
+		return (-1);
+
+	if (tstl_add_left(&ts, left) == -1)
+		return (-1);
+	if (tstl_add_right(&ts, right) == -1)
+		return (-1);
+
+	if (tstl_decide(&ts) == -1)
+		return (-1);
 
 	return (-1);
 }