public BNode(String nodeId){
  this.nodeId=nodeId;
  inputNodes=new HashMap<String,BNode>();
  outputNodes=new HashMap<String,BNode>();
}

public BNode ( String nodeId ) { this . nodeId = nodeId ; inputNodes = new HashMap < String , BNode > ( ) ; outputNodes = new HashMap < String , BNode > ( ) ; }
creates a node with a unique identifier , and a empty set of incoming nodes
