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:   (:import-from :bordeaux-threads)
    (:import-from :alexandria #:parse-body)
    (:export #:shell-pipeline #:shell-not #:& #:with-subshell #:shell-when
             #:shell-unless #:shell-if #:shell-while #:shell-for #:loop-break
             #:loop-continue #:shell-run))
  (in-package :shcl/core/shell-form)
  
  (optimization-settings)
  
negative.code:     :reader loop-jump-exit-info)))
  
  (define-condition loop-break (loop-jump)
    ()
    (:documentation
     "A condition representing that the an enclosing loop should stop
  running."))
  
  (define-condition loop-continue (loop-jump)
    ()
    (:documentation
     "A condition representing that an enclosing loop should skip ahead
  to the next iteration."))
  
  (defmacro jump-level (name &body body)
    (let ((block-label (gensym (symbol-name name)))
  