Take two possible (Borel) subsets of . Call them A and
B.
A could be point x=0.
B could be point x=1 only.
Generally, Y is divided into four subspaces by A and B, and
following the chalk dust method---imagine N experiments, which give
the following number of occurrences for events to fall in the four
subspaces.
. Call them A and
B.
for events to fall in the four
subspaces.
is number of events falling in subspace, S.