<itemvalue="{@template cloud_firestore.collection_reference.doc} Returns a `DocumentReference` with the provided path. If no [path] is provided, an auto-generated ID is used. The unique key generated is prefixed with a client-generated timestamp so that the resulting list will be chronologically-sorted. {@endtemplate}"/>
<itemvalue="Whether the collection contains an element equal to [element]. This operation will check each element in order for being equal to [element], unless it has a more efficient way to find an element equal to [element]. The equality used to determine whether [element] is equal to an element of the iterable defaults to the [Object.==] of the element. Some types of iterable may have a different equality used for its elements. For example, a [Set] may have a custom equality (see [Set.identity]) that its `contains` uses. Likewise the `Iterable` returned by a [Map.keys] call should use the same equality that the `Map` uses for keys."/>
<itemvalue="The value for the given [key], or `null` if [key] is not in the map. Some maps allow `null` as a value. For those maps, a lookup using this operator cannot distinguish between a key not being in the map, and the key being there with a `null` value. Methods like [containsKey] or [putIfAbsent] can be used if the distinction is important."/>
<itemvalue="Whether this map contains the given [key]. Returns true if any of the keys in the map are equal to `key` according to the equality used by the map. ```dart final moonCount = <String, int>{'Mercury': 0, 'Venus': 0, 'Earth': 1, 'Mars': 2, 'Jupiter': 79, 'Saturn': 82, 'Uranus': 27, 'Neptune': 14 }; final containsUranus = moonCount.containsKey('Uranus'); true final containsPluto = moonCount.containsKey('Pluto'); false ```"/>
