End
limit children until endKey: the values of data nodes sent by subscriptions will be limited to the last limit child nodes (or all child nodes if limit is not set) of the actual data nodes whose keys are lesser than or equal endKey.
Child nodes are ordered with respect to their keys in a dedicated lexicographic order. For example, if the actual data node to send is {a:9, b:8, c:7}, the EndAt("cba", 2) constraint will send {b:8, c:7} to notification targets.