public void extend(RuleGrounding other){
  if (other.isFailed()) {
    groundings.clear();
    return;
  }
  extend(other.getAlternatives());
}

public void extend ( RuleGrounding other ) { if ( other . isFailed ( ) ) { groundings . clear ( ) ; return ; } extend ( other . getAlternatives ( ) ) ; }
extends the existing groundings with the alternative groundings
