If a grammar is not in single-initial form, dual-initial rules can be converted to single-initial form by adding an empty span to B that combines with the first spans of C immediately to its left, as shown in Figure 2.
for each dual-initial rule A → B C, if the first span of C appears between spans i and i + 1 of B, create a new nonterminal B with ϕ(B ) = ϕ(B) + 1, and add a rule B → B, where B produces B along with a span of length zero between spans i and i + 1 of B.