public void removeMother(Node aMother){
  if (!motherToIndex.containsKey(aMother))   throw new IllegalArgumentException("The given node is not a mother of this node");
  motherToIndex.remove(aMother);
  mothers.remove(aMother);
}

public void removeMother ( Node aMother ) { if ( ! motherToIndex . containsKey ( aMother ) ) throw new IllegalArgumentException ( "The given node is not a mother of this node" ) ; motherToIndex . remove ( aMother ) ; mothers . remove ( aMother ) ; }
remove the given node from the list of mothers .
