anchor.diff:    (:import-from :alexandria)
     (:import-from :closer-mop)
     (:import-from :shcl/core/shell-lambda
  -                #:command-error #:shell-lambda #:handle-command-errors)
  +                #:command-error #:shell-lambda #:handle-command-errors
  +                #:wrap-errors)
     (:import-from :shcl/core/fd-table
                   #:with-fd-streams #:with-private-fds #:linearize-fd-bindings
                   #:fd-wrapper-value)
  
positive.code:       (values))))
  
  (defun lookup-command (name)
    (multiple-value-bind (value found) (fset:lookup (command-namespace-table *command-namespace*) name)
      (if (and found (not (fset:empty? value)))
          (nth-value 1 (fset:at-rank value 0))
  
negative.code: (defun register-special-builtin (name handler)
    (install-command name (make-instance 'special-builtin :handler handler)))
  
  (define-data builtin (command)
    ((handler
      :initarg :handler
      :initform (required)
      :updater builtin-handler))
    (:documentation
     "Instances of this class represent normal, non-special builtin
  commands.
  
  See `special-builtin'.  Builtins are like normal commands except they
  are implemented in the shell itself."))
  
  (defmethod documentation ((command builtin) (doc-type (eql t)))
    (documentation (builtin-handler command) doc-type))
  