CatsRepo.kt

import java.util.UUID

data class Cat(
    val id: UUID,
    val name: String,
    val ownerId: UUID
)

interface CatsRepo {
    operator fun get(id: UUID): Cat?
    fun listForOwner(ownerId: UUID): List<Cat>
    operator fun plusAssign(cat: Cat)
    operator fun minusAssign(id: UUID)
}