; See if the input starts with a given symbol.

(define (match-symbol input pattern)

  (cond ((null? (remain input)) #f)

    ((eqv? (car (remain input)) pattern) (r-cdr input))

    (else #f)))

​

; Allow the input to start with one of a list of patterns.

(define (match-or input pattern)

  (cond ((null? pattern) #f)

    ((match-pattern input (car pattern)))

    (else (match-or input (cdr pattern)))))

​

; Allow a sequence of patterns.

(define (match-seq input pattern)

  (if (null? pattern)

      input

      (let ((match (match-pattern input (car pattern))))

    (if match (match-seq match (cdr pattern)) #f))))

​

; Match with the pattern but no problem if it does not match.

(define (match-opt input pattern)

  (let ((match (match-pattern input (car pattern))))

    (if match match input)))

​

; Match anything (other than '()), until pattern is found. The rather

; clumsy form of requiring an ending pattern is needed to decide where

; the end of the match is. If none is given, this will match the rest

; of the sentence.

(define (match-any input pattern)

  (cond ((null? (remain input)) #f)

    ((null? pattern) (f-cons (remain input) (clear-remain input)))

    (else