code: private void parseIfExpr() throws TTXPathException {
  
          // parse if expression
          consume("if", true);
          consume(TokenType.OPEN_BR, true);
  
          // parse test expression axis
          parseExpression();
          consume(TokenType.CLOSE_BR, true);
  
          // parse then expression
          consume("then", true);
          parseExprSingle();
  
          // parse else expression
          consume("else", true);
          parseExprSingle();
  
          mPipeBuilder.addIfExpression(getTransaction());
  
      }
docs: Parses the the rule IfExpr according to the following production rule:
  <p>
  [7] IfExpr ::= <"if" "("> Expr ")" "then" ExprSingle "else" ExprSingle.
  </p>
  
  @throws TTXPathException