fold-left by Xif

(define (fold-left op base lst)
  (define (loop op so-far lst)
    (cond
     ((equal? lst '()) ; reached end of list, return so-far
      so-far)
     (else
      (loop op
            (op so-far (car lst))
            (cdr lst)
            ))))
  (loop op base lst)
  )