The convex continuation of probability charges P and Q on (X,A) is the probabilty charge which assigns to B in A the number
* aP(B)+(1-a)Q(B)
a in [0,1].
If P,Q simple => aP(B)+(1-a)Q(B) simple.
If Ps=the set of all simple probability charges on (X,A) Ps is closed under
convex continuation.