code: function(path, value, data , op, computed){
      var computed = this.computed,
        op = op || "=", prev, 
        computedProperty = computed? computed[path]:null;
  
      if(op !== '='){
        prev = computedProperty? computedProperty.get(this): data[path];
        switch(op){
          case "+=":
            value = prev + value;
            break;
          case "-=":
            value = prev - value;
            break;
          case "*=":
            value = prev * value;
            break;
          case "/=":
            value = prev / value;
            break;
          case "%=":
            value = prev % value;
            break;
        }
      }
      if(computedProperty) {
        if(computedProperty.set) return computedProperty.set(this, value);
      }
      data[path] = value;
      return value;
    }
docs: simple accessor set