docs: function sequence(streamProviders) {
      const result = es.through();
      function pop() {
          if (streamProviders.length === 0) {
              result.emit('end');
          }
          else {
              const fn = streamProviders.shift();
              fn()
                  .on('end', function () { setTimeout(pop, 0); })
                  .pipe(result, { end: false });
          }
      }
      pop();
      return result;
  }