Suppose S is a set of points in a metric space (X,d).
A point
x in S is an outer-point of S if there is a y in X - S such that
x is the closest point of S to y. Thus there must be a y in X - S
such that d(S,y) = d(x,y).
If the outer-points of S are in SO,
then the inner-points are SI=S-SO.
