code: function (cb) {
              if (cb) {
                  if (isPromise(cb)) {
                      cb = cb.callback;
                  }
                  if (this.__fired && this.__results) {
                      this.__callNextTick(cb, this.__results);
                  } else {
                      this.__cbs.push(cb);
                  }
              }
              return this;
          }
docs: Add a callback to the callback chain of the promise
  
  
  @param {Function|comb.Promise} cb the function or promise to callback when the promise is resolved.
  
  @return {comb.Promise} this promise for chaining