code: public static TreeGraphNode getSubject(TreeGraphNode t) {
      TreeGraphNode subj = getNodeInRelation(t, NOMINAL_SUBJECT);
      if (subj != null) {
        return subj;
      }
      subj = getNodeInRelation(t, CLAUSAL_SUBJECT);
      if (subj != null) {
        return subj;
      } else {
        return getNodeInRelation(t, NOMINAL_PASSIVE_SUBJECT);
      }
    }
docs: Tries to return a node representing the <code>SUBJECT</code> (whether
  nominal or clausal) of the given node <code>t</code>. Probably, node
  <code>t</code> should represent a clause or verb phrase.
  
  @param t
  a node in this <code>GrammaticalStructure</code>
  @return a node which is the subject of node <code>t</code>, or else
  <code>null</code>