Two strings γ 1 , γ 2 ∈ V * I are synchronous if each index from N occurs at most once in γ 1 and at most once in γ 2 , and index(γ 1 ) = index(γ 2 ).
γ 1 , γ 2 have the general form: ∈ I(N ), t i = t j for i = j, and π is a permutation of the set {1, .