code: function addPart(aType, aValue) {
        var ucname = aType.toUpperCase();
        if (ucname in this.parts) {
          this.parts[ucname].push(aValue);
        } else {
          this.parts[ucname] = [aValue];
        }
      }
docs: Adds a component (part) to the recurrence rule. This is not a component
  in the sense of {@link ICAL.Component}, but a part of the recurrence
  rule, i.e. BYMONTH.
  
  @param {String} aType            The name of the component part
  @param {Array|String} aValue     The component value