P a simple charge and f:X->R => expected value of f w.r.t. P, 
 
 * Ep(f)=Sum(X in support(P), f(x)P(x))

If X is a random variable:
 * Ex(f)=Sum(a in support(P), f(a)Px(a)) with f:X->R
 * Ex=Sum(a in support(P), aPx(a))