code: def combine_sources(sources, chunksize=None):
      r
      from pyemma.coordinates.data.sources_merger import SourcesMerger
      return SourcesMerger(sources, chunk=chunksize)
docs: r""" Combines multiple data sources to stream from.
  
      The given source objects (readers and transformers, eg. TICA) are concatenated in dimension axis during iteration.
      This can be used to couple arbitrary features in order to pass them to an Estimator expecting only one source,
      which is usually the case. All the parameters for iterator creation are passed to the actual sources, to ensure
      consistent behaviour.
  
      Parameters
      ----------
      sources : list, tuple
          list of DataSources (Readers, StreamingTransformers etc.) to combine for streaming access.
  
      chunksize: int, default=None
          Number of data frames to process at once. Choose a higher value here,
          to optimize thread usage and gain processing speed. If None is passed,
          use the default value of the underlying reader/data source. Choose zero to
          disable chunking at all.
  
      Notes
      -----
      This is currently only implemented for matching lengths trajectories.
  
      Returns
      -------
      merger : :class:`SourcesMerger <pyemma.coordinates.data.sources_merger.SourcesMerger>`