This difference in the definition of deterministic and non-deterministic machines also necessitates a change in the definition of what it means for a machine to decide a language \(X\). Recall that for a deterministic machine \(T\), a computation sequence starting from an initial configuration \(C_0\) is a finite or infinite sequence of machine configurations \(C_0,C_1,C_2, \ldots\) Such a configuration consists of a specification of the contents of \(T\)’s tape, internal state, and head position. \(C_{i+1}\) is the unique configuration determined by applying the transition function \(\delta\) to the active state-symbol pair encoded by \(C_i\) and is undefined if \(\delta\) is itself undefined on this pair (in which case the computation sequence is finite, corresponding to a halting computation). If \(N\) is a non-deterministic machine, however, there may be more than one configuration which is related to the current configuration by \(\Delta\) at the current head position. In this case, a finite or infinite sequence \(C_0,C_1,C_2, \ldots\) is said to be a computation sequence for \(N\) from the initial configuration \(C_0\) just in case for all \(i \geq 0\), \(C_{i+1}\) is among the configurations which are related by \(\Delta\) to \(C_i\) (and is similarly undefined if no such configuration exists).