To see the effect of this constraint, consider the following example with a source sentence s 1 s 2 and a shallow-1 grammar defined by these four rules: There are two derivations R 1 R 2 and R 1 R 3 R 4 which yield identical translations.
r 2 would not be allowed under the constraint introduced here because it does not rewrite an X 1 to an X 0 .