The structure of a collection dictates which is the easiest way to
divide up the collection. An array can be divided into a <EM>front</EM>
and a <EM>back</EM>, a stack into a <EM>top</EM> and a <EM>remainder</EM>.

<P><B>To add up the numbers on a stack:</B>
<UL>
  <LI>If the stack is empty, by convention the sum of no numbers is 0.
  <LI>If the stack is not empty, divide it into a Top and a Remainder.
      <UL>
        <LI>Top is a number: S1 = Top
        <LI>Remainder is a stack: add up its numbers; this is a
            recursive call returning S2.
        <LI>Combine: S = S1 + S2 = Top + sum of numbers in Remainder.
      </UL>
</UL>