diff --git a/lib/construct.c b/lib/construct.c
index 965beae..3eb4811 100644
--- a/lib/construct.c
+++ b/lib/construct.c
@@ -39,6 +39,10 @@ static void construct_term(const regex_term_t *term, fsa_t *out)
 
 static void concat_fsas(fsa_t *base, const fsa_t *other)
 {
+	// TODO: Handle the other's final state having transition rules.
+	assert(0 == other->states[0].count);
+
+	// Copy states other than the final state (index zero) to base.
 	const int new_count = base->count + other->count - 1;
 	if (base->capacity < new_count) {
 		do
@@ -47,23 +51,25 @@ static void concat_fsas(fsa_t *base, const fsa_t *other)
 		base->states = realloc(base->states, base->capacity);
 		assert(base->states);
 	}
-
 	const int copy_size = (other->count - 1) * sizeof(fsa_state_t);
 	memcpy(&base->states[base->count], &other->states[1], copy_size);
 
-	const int id_offset = base->count - 1;
+	// Retarget new states' rules.
 	for (int i = base->count; i < new_count; ++i) {
 		fsa_state_t *state = &base->states[i];
 		for (int j = 0; j < state->count; ++j) {
 			if (0 == state->rules[j].next)
 				state->rules[j].next = base->initial;
 			else
-				state->rules[j].next += id_offset;
+				// States' indices have increased by one less than the
+				// base count, as the final state came before them and
+				// was not copied.
+				state->rules[j].next += base->count - 1;
 		}
 	}
 
+	base->initial = other->initial + base->count - 1;
 	base->count = new_count;
-	base->initial = other->initial + id_offset;
 
 	free(other->states[0].rules);
 	free(other->states);