  Prefix-free encodings are useful for a number of practical purposes, and they turn out to be useful in defining randomness also. (As we will see in  §2.3,  they are of special importance in avoiding a problem in the definition of infinite Kolmogorov random sequences.) The change is the natural one: we appeal, not to the plain complexity of a sequence in defining its randomness, but the so-called prefix-free complexity (Downey and Hirschfeldt 2010: §2.5ff; Dasgupta 2011: §8).