= Solution
Use one real <linked list node> per occurrence, with a private reference to the first:
``
class Node(val value: Int, var next: Node)
// Inside the queue implementation:
private var first: Node = null
``
The <representation invariant> says that the reachable chain is finite, acyclic and nondecreasing in `value`, and is owned by the queue. Equal adjacent values are explicitly allowed. The <abstraction function> is the <multiset> containing one occurrence of `v.value` for every reachable real <linked list node> `v`. Initially `first==null` represents the empty <multiset>. No artificial smallest `Int` is needed as a sentinel, and `Int.MinValue` remains a valid real value.
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