in scala-repl/src/main/scala/org/jetbrains/ztools/scala/core/TrieMap.scala [38:42]
def put(key: String, node: TrieMap.Node[T]): Option[Node[T]] = {
if (children == null)
children = mutable.Map[String, TrieMap.Node[T]]()
children.put(key, node)
}