Past exam of the year 1 of the computer science course of the University of Oxford / 2021 / Imperative programming (resit) / 4 / a

Use a finite sequence $q$ as abstract state, initially empty. One Scala interface isThe emptiness query has no precondition; its result is $q_0=⟨⟩$ and its postcondition is $q=q_0$. Adding on the left has no precondition and gives $q=⟨x⟩++q_0$. Adding on the right gives $q=q_0++⟨x⟩$. Both return Unit.

Each removal requires a nonempty incoming sequence. getLeft() returns its first entry and leaves the remaining final sequence; getRight() returns its last entry and leaves the remaining initial sequence. These contracts remove exactly one occurrence, so duplicate values are permitted and do not collapse the sequence into a set.