Function: (SETF LOOKUP)

Source

(defun (setf lookup) (value environment type name &key (error-p nil))
  (loop
     for env-piece in environment
     when (and (eql (first env-piece)  type)
               (eql (second env-piece) name))
       do (setf (cddr env-piece) value) and
       return value
     finally
       (when error-p
         (error "Sorry, No value for ~S of type ~S in environment ~S found."
                name type environment))))