public void addSource(Fact other){
  if (derivation == null) {
    derivation=new Derivation();
  }
  derivation.addSource(other);
}

public void addSource ( Fact other ) { if ( derivation == null ) { derivation = new Derivation ( ) ; } derivation . addSource ( other ) ; }
specify a source . wrapper for derivation . addsource . instantiates a derivation if none exists .
