<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _copy_across(self, rel_path, cb=None):
 """If the upstream doesn't have the file, get it from the alternate and store it in the upstream"""


        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()