The user-defined (stair) is the discrete alternative of continuous User-defined function.
The Stair function is defined through a discrete set of values Xi, i = 1…N and associated probabilities S(Xi).
This is the pair of set (Xi, S(Xi)) where Xi represents all possible values that might be assigned to
the variable, whereas S(Xi) represents their cumulative probabilities.
Such a distribution will be entered in form of a list of couples (Xi, S(Xi)). Entering such a distribution
in cumulated form allow to control that the sum of the probabilities is equal to 1.
The definition range of the input Cum. Prob. (S(Xi)) is (0, 1)