Past exam of the year 1 of the computer science course of the University of Oxford / 2019 / Functional programming, design and analysis of algorithms / 4 / a / Solution

For proper lazy lists, the generic condition is a finite first list, or an empty second list. A finite first list reaches its terminating constructor, after which list concatenation exposes the second one. An infinite first list never yields control to the second; it therefore cannot deliver occurrences originating there unless there are none. This concerns input occurrences, rather than an accidental overlap of the two inputs' value sets.

A first spine ending in the bottom value also prevents access to the second list. Such inputs require the partial-input qualification in part c; a visible prefix is different from a terminating finite list.