= Solution
Use an immutable <hash map> for the concrete value, omitting zero-count entries:
``
class CompleteMultiSetImp[T](initial: Map[T, Int]) extends CompleteMultiSet[T] {
private val table = initial.filter { case (_, n) => n != 0 }
override def count(t: T): Int = table.getOrElse(t, 0)
override lazy val present: Seq[T] = table.keys.toVector
}
def cms[T](ts: T*): CompleteMultiSet[T] =
new CompleteMultiSetImp[T](ts.distinct.map(t => (t, 1)).toMap)
``
The <abstraction function> is the map with implicit zero outside its keys. Immutable storage makes saved values independent of later operations, and the inherited defaults dispatch through the overridden `count` and `present`. A hash lookup has expected constant cost under ordinary hashing assumptions, with collision-dependent worst cases.
\b[Repeated arguments still get count one], because the constructor first removes duplicates. All other elements have count zero, satisfying the intended stronger reading of the constructor specification. Treating `ts` as a bag and adding one for every repetition would implement a different operation.
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