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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 format_cffi_externs(cls):
 """Generate stubs for the cffi bindings from @_extern_decl methods."""

    extern_decls = [
      f.extern_signature.pretty_print()
      for _, f in cls._extern_fields.items()
    ]
    return (
      'extern "Python" {\n'
      + '\n'.join(extern_decls)
      + '\n}\n')