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Answer to Question 31A-2

Here is an evaluation using the following equations.
(sum.1)   sum([ ]) = 0  
(sum.2)   sum(L) = head(L) + sum(tail(t))    (when L is not empty)

Evaluation is as follows.

   sum([3,5,7])
     = 3 + sum([5,7])  by (sum.2)
     = 3 + (5 + sum([7]))  by (sum.2)
     = 3 + (5 + (7 + sum([ ])))  by (sum.2)
     = 3 + (5 + (7 + 0))  by (sum.1)
     = 15  by arithmetic