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+* status
+  Excercise 1.1 - 1.5
+* notes
+** 1
+*** 1
+**** 5
+#+BEGIN_QUOTE
+In general, when modeling phenomena in science and engineering, we
+begin with simplified, incomplete models. As we examine things in
+greater detail, these simple models become inadequate and must be
+replaced by more refined models.
+#+END_QUOTE
+* exercises
+** 1
+*** 4
+
+    #+BEGIN_SRC scheme
+    (define (a-plus-abs-b a b)
+      ((if (> b 0) + -) a b))
+    #+END_SRC
+
+If ~b~ is greater than 0, do ~a + b~; otherwise do ~a - b~.
+*** 5
+
+code at [[./one/five.scm]]
+
+If the interpreter uses *applicative order* to evaluate the
+expression:
+
+#+BEGIN_SRC scheme
+(test 0 (p))
+#+END_SRC
+
+The parameters are evaluated before applying the compound procedure
+~test~; 0 evaluates to 0, ~(p)~ never finishes evaluating as the
+compound procedure ~p~ recursively calls itself again and again
+infinitely.
+
+If same expression is evaluated by the interpreter using *normal
+order*, the expression will be expanded to
+
+#+BEGIN_SRC scheme
+  (if (= 0 0)
+      0
+      (p)))
+#+END_SRC
+
+and will evaluate to ~0~.