code: def _copy_across(self, rel_path, cb=None):
          
  
          from . import copy_file_or_flo
  
          if not self.upstream.has(rel_path):
  
              if not self.alternate.has(rel_path):
  
                  return None
  
              source = self.alternate.get_stream(rel_path)
  
              sink = self.upstream.put_stream(rel_path, metadata=source.meta)
  
              try:
                  copy_file_or_flo(source, sink, cb=cb)
              except:
                  self.upstream.remove(rel_path, propagate=True)
                  raise
  
              source.close()
              sink.close()
docs: If the upstream doesn't have the file, get it from the alternate and store it in the upstream