class TagsTest {
  @Test
  fun emptyTags() {
    val tags = EmptyTags
    assertThat(tags[String::class]).isNull()
  }

  @Test
  fun singleElement() {
    val tags = EmptyTags.plus(String::class, "hello")
    assertThat(tags[String::class]).isEqualTo("hello")
  }

  @Test
  fun multipleElements() {
    val tags =
      EmptyTags
        .plus(String::class, "hello")

import java.util.concurrent.atomic.AtomicReference
import kotlin.reflect.KClass

/**
 * An immutable collection of key-value pairs implemented as a singly-linked list.
 *
 * Build up a collection by starting with [EmptyTags] and repeatedly calling [plus]. Each such call
 * returns a new instance.
 *
 * This collection is optimized for safe concurrent access over a very small number of elements.
 *