anchor.diff:                  (car tail))))))
   
   (defmacro & (&body body)
  +  "Evaluate the given forms in the background."
     (declare (ignore body))
     (error 'not-implemented :feature "Background jobs"))
   
   (defmacro shell-if (condition then &optional (else nil else-p))
  +  "The shell version of `if'.
  +
  +First, `condition' is evaluated.  If it produces a truthy exit status,
  +then the `then' form is evaluated and the result is returned.  If it a
  +produces a falsey exit status, then this macro will evaluate and
  +return the result of `else'.  If an `else' form isn't provided, then
  +the return value of `condition' is produced."
     (let ((value (gensym "VALUE")))
       `(let ((,value ,condition))
          (if (exit-info-true-p ,value)
  
positive.code:         (funcall modify-environment))))
  
  (defmacro shell-run (argument-list &key environment-changes fd-changes)
    (let ((modify-environment (gensym "MODIFY-ENVIRONMENT"))
          (fd (gensym "FD"))
          (value (gensym "VALUE"))
  
negative.code:     (async-result-get (aref results (1- (length pipeline-fns))))))
  
  (defmacro shell-pipeline (&body body)
    "Evaluate each form of body where standard-output of each form is
  connected to standard-input of the next form."
    `(pipeline-fn
      ,@(mapcar (lambda (form)
                  `(lambda ()
  