solution to 3.38

diff --git a/src/sicp/ex3_38.rkt b/src/sicp/ex3_38.rkt
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+++ b/src/sicp/ex3_38.rkt
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+#lang racket
+
+#|
+
+Peter:         (set! balance (+ balance 10))
+Paul:  (set! balance (- balance 20))
+Mary:  (set! balance (- balance (/ balance 2)))
+
+1.
+
+a. Peter deposits $10
+b. Paul withdraws $20
+c. Mary withdraws half of balance.
+
+100 + 10 - 20 = 90 -> 90/2 -> $45
+
+2. 
+
+a. Peter deposits $10
+b. Mary withdraws half of balance.
+c. Paul withdraws $20
+
+100 + 10 -> 55 - 20 -> $35
+
+3.
+
+a. Paul withdraws $20
+b. Peter deposits $10
+c. Mary withdraws half of balance.
+
+100 - 20 + 10 -> 110 /2 -> $55
+
+4. 
+
+a. Paul withdraws $20
+b. Mary withdraws half of balance.
+c. Peter deposits $10
+
+100 - 20 => 80 / 2 => 40 + 10 => 50
+
+5.
+
+a. Mary withdraws half of balance.
+b. Peter deposits $10
+c. Paul withdraws $20
+
+100 / 2 => 50 + 10 - 20 => 40
+
+6.
+
+a. Mary withdraws half of balance.
+b. Paul withdraws $20
+c. Peter deposits $10
+
+100 / 2 => 50 - 20 + 10 => 40
+
+|#
+
+#|
+
+Balance could take many values if the three transactions are allowed to
+happen concurrently.
+
+It could be 110, 80 or 50 if one of them mutates the balance variable
+at the end depending on which one manages to finish last. It could also
+be that 2 of them work concurrently and the resultant value is processed
+by the third transaction.
+
+|#
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