anchor.diff:  (defstruct (parallel:stream
               (:constructor %make-stream))
     (index   0 :type (mod #.+chunk-size+))
  -  (chunk nil :type simple-vector)
  +  (chunk nil :type simple-vector :read-only t)
     (%next nil :type (or null function parallel:stream parallel:future)))
   
   (defun make-stream (index chunk %next)
  
positive.code:              (setf (stream-%next stream)
                     (future-value (stream-next stream)))))))
  
  
  (declaim (maybe-inline parallel:unfold
                         parallel:scan* parallel:scan parallel:foreach
  
negative.code:   (:use)
    (:export "PROMISE" "PROMISE-VALUE" "LET"
             "FUTURE" "FUTURE-VALUE" "FUTURE-VALUE*" "BIND"
             "STREAM" "HEAD" "TAIL" "TAIL!" "TAIL-PREFETCH" "UNFOLD" "SCAN" "SCAN*" "FOREACH" "FOLD"
             "DOTIMES" "MAP" "REDUCE"
             "MAP-GROUP-REDUCE"))
  
  