==> combinatorics/subsets.p <==
Out of the set of integers 1,...,100 you are given ten different
integers.  From this set, A, of ten integers you can always find two
disjoint non-empty subsets, S & T, such that the sum of elements in S
equals the sum of elements in T.  Note: S union T need not be all ten
elements of A.  Prove this.