spec: sequence protocol tests — 45 tests, all passing on JS and OCaml

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spec/evaluator.sx

@@ -3649,7 +3649,8 @@
   (fn
     (s)
     (cond
-      ((or (= s nil) (list? s)) (len s))
+      ((= s nil) 0)
+      ((list? s) (len s))
       ((vector? s) (vector-length s))
       ((string? s) (len s))
       (else (len (seq-to-list s))))))
@@ -3674,6 +3675,15 @@
       ((and (string? s1) (string? s2)) (str s1 s2))
       (else (concat (seq-to-list s1) (seq-to-list s2))))))
 
+(define
+  build-range
+  (fn
+    (i end step acc)
+    (if
+      (if (> step 0) (>= i end) (<= i end))
+      (reverse acc)
+      (build-range (+ i step) end step (cons i acc)))))
+
 (define
   in-range
   (fn
@@ -3686,16 +3696,7 @@
       (if
         (= step 0)
         (error "in-range: step cannot be zero")
-        (do
-          (define
-            build
-            (fn
-              (i acc)
-              (if
-                (if (> step 0) (>= i end) (<= i end))
-                (reverse acc)
-                (build (+ i step) (cons i acc)))))
-          (build real-start (list)))))))
+        (build-range real-start end step (list))))))
 
 (define
   step-ho-map