(defun riemann-left-fast (a b step)
  (declare (type double-float a b step)
           (optimize (speed 3)
                     (safety 0)))
  (let ((acc 0.0d0))
    (declare (type double-float acc))
    (loop for n double-float from a to (- b step) by step do
      (incf acc n))
    (* step acc)))