pc:diatonic   scheme

;; returns a chord following basic diatonic harmony rules
;; based on root (0 for C etc.) maj/min ('^ or '-) and degree (i-vii)
(define pc:diatonic
  (lambda (root maj-min degree)
    (if (number? degree)
        (set! degree (cdr (assoc degree '((0 . i) (1 . ii) (2 . ii)
                                          (3 . iii) (4 . iii) (5 . iv)
                                          (6 . iv) (7 . v) (8 . vi) (9 . vi)
                                          (10 . vii) (11 . vii))))))
    (let ((val (assoc degree
                      (if (equal? '^ maj-min)
                          *pc:diatonic-major*
                          *pc:diatonic-minor*))))
      (pc:chord (modulo (+ root (cadr val)) 12) (cddr val)))))