code: private void parseCastableExpr() throws TTXPathException {
  
          parseCastExpr();
          if (is("castable", true)) {
  
              consume("as", true);
              mPipeBuilder.addCastableExpr(getTransaction(), parseSingleType());
  
          }
      }
docs: Parses the the rule CastableExpr according to the following production
  rule:
  <p>
  [18] CastableExpr ::= CastExpr ( <"castable" "as"> SingleType )? .
  </p>
  
  @throws TTXPathException