code: private void parseTreatExpr() throws TTXPathException {
  
          parseCastableExpr();
          if (is("treat", true)) {
              consume("as", true);
              mPipeBuilder.addTreatExpr(getTransaction(), parseSequenceType());
          }
      }
docs: Parses the the rule TreatExpr according to the following production rule:
  <p>
  [17] TreatExpr ::= CastableExpr ( <"treat" "as"> SequenceType )? .
  </p>
  
  @throws TTXPathException