  We develop a theory of probabilistic coherence spaces equipped with an
additional extensional structure and apply it to approximating probability of
convergence of ground type programs of probabilistic PCF whose free variables
are of ground types. To this end we define an adapted version of Krivine
Machine which computes polynomial approximations of the semantics of these
programs in the model. These polynomials provide approximations from below and
from above of probabilities of convergence; this is made possible by extending
the language with an error symbol which is extensionally maximal in the model.
