code: private void parseUnionExpr() throws TTXPathException {
  
          parseIntersectExceptExpr();
  
          while (is("union", true) || is(TokenType.OR, true)) {
  
              // parse second operand axis
  
              mPipeBuilder.addExpressionSingle();
  
              parseIntersectExceptExpr();
  
              mPipeBuilder.addUnionExpression(getTransaction());
  
          }
      }
docs: Parses the the rule UnionExpr according to the following production rule:
  <p>
  [14] UnionExpr ::= IntersectExceptExpr ( ("union" | "|") IntersectExceptExpr )* .
  </p>
  
  @throws TTXPathException