public AllocationExplanation add(ShardId shardId,NodeExplanation nodeExplanation){
  List<NodeExplanation> list=explanations.get(shardId);
  if (list == null) {
    list=new ArrayList<>();
    explanations.put(shardId,list);
  }
  list.add(nodeExplanation);
  return this;
}

public AllocationExplanation add ( ShardId shardId , NodeExplanation nodeExplanation ) { List < NodeExplanation > list = explanations . get ( shardId ) ; if ( list == null ) { list = new ArrayList < > ( ) ; explanations . put ( shardId , list ) ; } list . add ( nodeExplanation ) ; return this ; }
create and add a node explanation to this explanation referencing a shard
