 To illustrate how such functions are Turing computable, we discuss one such function in more detail, viz. the compare function. It is constructed on the basis of a number of other Turing computable functions which are built on top of each other. In order to understand how these functions work, remember that Turing used a system of alternating F and E-squares where the F-squares contain the actual quintuples and complete configurations and the E-squares are used as a way to mark off certain parts of the machine tape. For the comparing of two sequences \(S_1\) and \(S_2\), each symbol of \(S_1\) will be marked by some symbol a and each symbol of \(S_2\) will be marked by some symbol b.