public void addArc(String sParent,String sChild) throws Exception {
  int nParent=getNode(sParent);
  int nChild=getNode(sChild);
  addArc(nParent,nChild);
}

public void addArc ( String sParent , String sChild ) throws Exception { int nParent = getNode ( sParent ) ; int nChild = getNode ( sChild ) ; addArc ( nParent , nChild ) ; }
add arc between two nodes distributions are updated by duplication for every value of the parent node .
