Challenge 2: mySum/2 Wrapping the Accumulator Version — Possible Solution
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list_utils.pl:

mySum([], Acc, Acc).
mySum([H|T], Acc, Sum) :- Acc1 is Acc + H, mySum(T, Acc1, Sum).

mySum(List, Sum) :- mySum(List, 0, Sum).   -- the public 2-argument wrapper

Query:

    ?- mySum([4, 8, 15, 16, 23], Total).
    Total = 66.

Explanation:
mySum/2 simply calls mySum/3 with an initial accumulator of 0 --
note this is a genuinely SEPARATE predicate from mySum/3, per the
chapter's own name+arity rule (prolog1-2): mySum/2 and mySum/3 are
different predicates that happen to share a name. Each recursive call
in mySum/3 adds the current head to the running accumulator (Acc1 is
Acc + H) and passes that updated total forward to the next call. Once
the list is exhausted, the base case mySum([], Acc, Acc) unifies the
final Sum with whatever the accumulator has become by that point --
66, the correct total of all five numbers.

WHY THIS WORKS AS AN ANSWER
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This wraps the chapter's own three-argument accumulator predicate in
a genuine two-argument public interface, correctly treating mySum/2
and mySum/3 as separate predicates per prolog1-2's own name+arity
rule, and confirms the correct sum over a real list.