diff --git a/lib/fiber.sx b/lib/fiber.sx
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+; lib/fiber.sx — pure SX fiber library using call/cc
+;
+; A fiber is a cooperative coroutine with true suspension (no eager
+; pre-execution). Each fiber is a dict {:resume fn :done? fn}.
+;
+; make-fiber body   → fiber dict
+;   body = (fn (yield init-val) ...)  — body receives yield + first resume val
+;   yield = (fn (val) ...)            — suspends fiber, returns val to resumer
+;
+; fiber-resume f v  → next yielded value, or nil when body returns
+; fiber-done? f     → true after body has returned
+
+(define make-fiber
+  (fn (body)
+    (let
+      ((resume-k nil)
+        (caller-k nil)
+        (done false))
+      (let
+        ((yield
+            (fn (val)
+              (call/cc
+                (fn (k)
+                  (set! resume-k k)
+                  (caller-k val))))))
+        {:resume
+         (fn (val)
+           (if
+             done
+             nil
+             (call/cc
+               (fn (k)
+                 (set! caller-k k)
+                 (if
+                   (nil? resume-k)
+                   (begin
+                     (body yield val)
+                     (set! done true)
+                     (k nil))
+                   (resume-k val))))))
+         :done? (fn () done)}))))
+
+(define fiber-resume (fn (f v) ((get f :resume) v)))
+(define fiber-done?  (fn (f) ((get f :done?))))