{ //initializer
   var util = require('util');
   console.log(process.cwd());
   var PEGStack = require('@ull-esit-pl/peg-stack');
   console.log('PEGStack = '+util.inspect(PEGStack));
   var stack = new PEGStack();
   var action = function() {
     var [val1, op, val2] = stack.pop(3);
     stack.push(eval(val1+op+val2)); 
   }
 }

sum     = first:product &{ return stack.push(first); } 
            (op:[+-] product:product 
            &{ stack.push(op, product); return stack.make(action); })* 
            { return stack.pop(); }