code: function (callback, errback) {
              if (isPromise(callback)) {
                  errback = callback.errback;
                  callback = callback.callback;
              }
              this.addCallback(callback);
              this.addErrback(errback);
  
              return this;
          }
docs: Call to specify action to take after promise completes or errors
  
  @param {Function} [callback=null] function to call after the promise completes successfully
  @param {Function} [errback=null] function to call if the promise errors
  
  @return {comb.Promise} this promise for chaining