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;; I get:
;;;
;;; Warning (#<thread: thread1>): in thread: (mutex-unlock) Internal scheduler error: unknown thread state
;;; #<thread: primordial>
;;; ready
;;;
;;; on C4.13 and C5, csc and csi
;;;
;;; why?
;;;
(cond-expand
 (chicken-5 (import (chicken process signal) srfi-18))
 (else      (use posix srfi-18)))

(define m (make-mutex))

(thread-start!
 (lambda ()
   (let loop ()
     (mutex-lock! m)
     (thread-yield!)     
     (mutex-unlock! m)
     (loop))))

(let loop ()
  (if (mutex-lock! m 0)
      (begin (thread-yield!)
             (mutex-unlock! m)
             (loop))
      (begin (thread-yield!)
             (loop))))