If we write the jth rule in this sequence as A j → ν j , there must exist a synchronous rule in G of the form A j → ν j , ν j that rewrites the same nonterminal.
g licenses a synchronous derivation (A 1 , A 1 ) ⇒ * (βA 1 δ, β A 1 δ ) for some β and δ .