formula_2 is the set of all infinite, binary sequences. We can view a real number in the unit interval as an infinite binary sequence, by considering its binary expansion. We may also view a language (a set of binary strings) as an infinite binary sequence, by setting the "n"th bit of the sequence to 1 if and only if the "n"th binary string (in lexicographical order) is contained in the language. Thus, sets of real numbers in the unit interval and complexity classes (which are sets of languages) may both be viewed as sets of infinite binary sequences, and thus the techniques of measure theory used to measure the size of sets of real numbers may be applied to measure complexity classes. However, since each computable complexity class contains only a countable number of elements(because the number of computable languages is countable), each complexity class has Lebesgue measure 0. Thus, to do measure theory inside of complexity classes, we must define an alternative measure that works meaningfully on countable sets of infinite sequences. For this measure to be meaningful, it should reflect something about the underlying definition of each complexity class; namely, that they are defined by computational problems that can be solved within a given resource bound.