code: private void parseExprSingle() throws TTXPathException {
  
          mPipeBuilder.addExpressionSingle();
  
          final String tContent = mToken.getContent();
          if ("for".equals(tContent)) {
              parseForExpr();
          } else if ("some".equals(tContent) || "every".equals(tContent)) {
              parseQuantifiedExpr();
          } else if ("if".equals(tContent)) {
              parseIfExpr();
          } else {
              parseOrExpr();
          }
      }
docs: Parses the the rule ExprSingle according to the following production
  rule:
  <p>
  [3] ExprSingle ::= ForExpr | QuantifiedExpr | IfExpr | OrExpr .
  </p>
  
  @throws TTXPathException