public void addSource(Fact predecessor){
  sources.add(predecessor);
  if (predecessor.isInference()) {
    sourceNodes.addAll(predecessor.getDerivation().sourceNodes);
  }
}

public void addSource ( Fact predecessor ) { sources . add ( predecessor ) ; if ( predecessor . isInference ( ) ) { sourceNodes . addAll ( predecessor . getDerivation ( ) . sourceNodes ) ; } }
record that a particular fact was used to derive this .
