code: public boolean isApplicable(Tree t) {
      // System.err.println("Testing whether " + sourcePattern + " matches " + ((TreeGraphNode) t).toOneLineString());
      return (sourcePattern != null) && (t.value() != null) &&
               sourcePattern.matcher(t.value()).matches();
    }
docs: Returns <code>true</code> iff the value of <code>Tree</code>
  node <code>t</code> matches the <code>sourcePattern</code> for
  this <code>GrammaticalRelation</code>, indicating that this
  <code>GrammaticalRelation</code> is one that could hold between
  <code>Tree</code> node <code>t</code> and some other node.