Add beginnings of Chapter 3 material.

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diff --git a/chapter-3/3.1-combining-arithmetics.scm b/chapter-3/3.1-combining-arithmetics.scm
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+;;; Copyright © 2021 Gerald Sussman and Chris Hanson
+;;; Copyright © 2022 David Thompson <dthompson2@worcester.edu>
+;;;
+;;; This program is free software: you can redistribute it and/or
+;;; modify it under the terms of the GNU General Public License as
+;;; published by the Free Software Foundation, either version 3 of the
+;;; License, or (at your option) any later version.
+;;;
+;;; This program is distributed in the hope that it will be useful,
+;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;; General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with this program.  If not, see
+;;; <http://www.gnu.org/licenses/>.
+
+(define (stormer-2 F h)
+  (lambda (history)
+    (+ (* 2 (x 0 history))
+       (* -1 (x 1 history))
+       (* (/ (expt h 2) 12)
+          (+ (* 13 (F (t 0 history) (x 0 history)))
+             (* -2 (F (t 1 history) (x 1 history)))
+             (F (t 2 history) (x 2 history)))))))
+
+(define (stepper h integrator)
+  (lambda (history)
+    (extend-history (+ (t 0 history) h)
+                    (integrator history)
+                    history)))
+
+(define n:> >)
+(define n:- -)
+
+(define (evolver F h make-integrator)
+  (let* ((integrator (make-integrator F h))
+         (step (stepper h integrator)))
+    (define (evolve history n-steps)
+      (if (n:> n-steps 0)
+          (evolve (step history) (n:- n-steps 1))
+          history))
+    evolve))