We will specify a computation of one of these automata by specifying the pair of the symbols that are in the window and the remainder of the string to the right of the window at each step of the computation.
De?nition 4 (Instantaneous Descriptions of SL2 Automata) An Instantaneous Description (ID) of a strictly 2-local automaton A = ( Σ,T) is a pair:
where pi is the pair of symbols currently in the window and wi is the suffx of the input that is on the tape to the right of the window.