Figure 3 contains a sample corpus from which four rules can be extracted: From Equation 4, the probabilities of r 3 and r 4 must be 1, and those of r 1 and r 2 must sum to 1.
the total probability mass, which is distributed across two possible output strings a'b'c' and b'a'c', is: It is relatively easy to prove that the probabilities of all derivations that correspond to a given decomposition λ i ∈ Λ sum to 1 (the proof is omitted due to constraints on space).