@Nullable private Ast find(@NotNull String id){
  Ast node=this;
  while (node != null && !id.equals(node.myId)) {
    node=node.myNext;
  }
  return node;
}

@ Nullable private Ast find ( @ NotNull String id ) { Ast node = this ; while ( node != null && ! id . equals ( node . myId ) ) { node = node . myNext ; } return node ; }
find an ast node with the specified id from the list of asts represented by the next chain . return null if no such node is found .
