code: def merge(data,skip=50,fraction=1.0):
      
  
      w,s,d = data.chains.shape
      
      start = int((1.0 - fraction) * s)
      total = int((s - start) / skip)
          
      return data.chains[:,start::skip,:].reshape((w*total,d))
docs: Merge one every 'skip' clouds into a single emcee population,
      using the later 'fraction' of the run.