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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _GetSourceFileSystem(self, source_path_spec, resolver_context=None):
 """Retrieves the file system of the source. Args: source_path_spec (dfvfs.PathSpec):
 source path specification of the file system. resolver_context (dfvfs.Context):
 resolver context. Returns: tuple: containing: dfvfs.FileSystem: file system. dfvfs.PathSpec: mount point path specification that refers to the base location of the file system. Raises: RuntimeError: if source path specification is not set. """

    if not source_path_spec:
      raise RuntimeError('Missing source.')

    file_system = path_spec_resolver.Resolver.OpenFileSystem(
        source_path_spec, resolver_context=resolver_context)

    type_indicator = source_path_spec.type_indicator
    if path_spec_factory.Factory.IsSystemLevelTypeIndicator(type_indicator):
      mount_point = source_path_spec
    else:
      mount_point = source_path_spec.parent

    return file_system, mount_point