rpn peg.js wrong version

rpn.pegjs.wrong

{
  function buildBinaryExpression(op, left, right) {
    return {
      type: 'BinaryExpression',
      operator: op,
      left: left,
      right: right
    }
  }
}

Expression
  = head:Term _ tail:Term _ op:Operator {
      return buildBinaryExpression(op, head, tail);
    }
  / Term

Term 
  = head:Factor _ tail:Factor _ op:Operator {
      return buildBinaryExpression(op, head, tail);
    }
  / Factor
  
Factor
  = Integer
  / Expression
    
Operator
  = "+" 
  / "-"
  / "*"
  / "/"

Integer "integer"
  = [0-9]+ {
    return parseInt(text(), 10); 
  }

_ "whitespace"
  = [ ]+