;;; -*- Mode: Lisp; Syntax: Common-Lisp; -*- File: language/parsing.lisp

;;;; Parsing Algorithms

(defvar *grammar* nil
  "The current default grammar.")

(defstruct (parse-node (:type list)
		       (:constructor parse-node (category &optional children)))
  "The parse-node data structure implied in Figure 22.7 [p 666]"
  category children)

(defun bottom-up-parse (words &optional (grammar *grammar*))
  "Return a forest of parses or failure. [p 666]"
  (let ((forest (mapcar #'parse-node words)))
    (loop
     (if (and (length=1 forest) (eq (parse-node-category (first forest))
				    (grammar-start-symbol grammar)))
	 (RETURN (first forest))
     (let* ((i