fun <K, V> contain(key: K, v: V): Matcher<Map<K, V>> = object : Matcher<Map<K, V>> {
   override fun test(value: Map<K, V>) = MatcherResult(
      value[key] == v,
      { "Map should contain mapping $key=$v but was ${buildActualValue(value)}" },
      { "Map should not contain mapping $key=$v but was $value" }
   )

   private fun buildActualValue(map: Map<K, V>) = map[key]?.let { "$key=$it" } ?: map
}