code: private void parseExpression() throws TTXPathException {
  
          mPipeBuilder.addExpr();
  
          int no = 0;
          do {
              parseExprSingle();
              no++;
  
          } while (is(TokenType.COMMA, true));
  
          mPipeBuilder.finishExpr(getTransaction(), no);
      }
docs: Parses an XPath expression according to the following EBNF production
  rule:
  <p>
  [2] Expr ::= ExprSingle ("," ExprSingle)* .
  </p>
  
  @throws TTXPathException