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+helper(in-package #:parse)
+
+(define-condition parser-error (error)
+  ((message :initarg :message
+            :initform nil
+            :reader message))
+  (:report (lambda (condition stream)
+             (format stream "~A" (message condition))))
+  (:documentation "Dedicated error for an invalid parse."))
+
+(defun tokens->ast (program)
+  "Given PROGRAM, which is a list of lists of symbols,
+filters out the labels and parses."
+  ;; TODO add directives
+  (let ((program (remove nil (mapcar #'extract-label program)))
+        (i 0))
+    (mapcar (lambda (l) (extract-instruction l i)) program)))
+
+(let ((i 0))
+  (defun extract-label (line)
+    "Given a series of tokens LINE, determines if LINE is
+in the form STRING {colon}. If it is, then it is treated as a
+label, and pushed onto the stack with the line index.
+
+Note that this function is intended to be called using mapcar,
+so that labels can be added to a map and otherwise removed from
+processing."
+    (trivia:match line
+      ((list (and id (type string))
+             (satisfies (lambda (x) (equal x 'lex::colon))))
+       (progn (push (cons (read-from-string id) i) util:label-loc) nil))
+      (_ (progn (incf i) line)))))
+
+(defun extract-instruction (line i)
+  "Given instruction LINE, determines the expected type format and passes
+LINE and the index I to the the respective function."
+  ;; TODO add pseudo-ops (i.e., nop, mov, ...)
+  (let* ((type-map '((r-type . extract-r-type)
+                     (i-type . extract-i-type)
+                     (j-type . extract-j-type)))
+         (keyword (car line))
+         (type-fn (cdr (assoc keyword type-map))))
+    (if type-fn
+        (funcall type-fn line i)
+        (error 'parser-error
+               (format nil "PARSE failed--~a is not a known keyword.~%" (keyword))))))
+
+(defun extract-r-type (line i)
+  'r)
+
+(defun extract-i-type (line i)
+  'i)
+
+(defun extract-j-type (line i)
+  'j)