'_' and '__'

[Mike McClain]

    In their discussion of 'List replication operator'
Steven D'Aprano and Ben Finney used these '_' and '__'.
Steve said, "[[] for _ in range(5)]".
Ben said, "[ [] for __ in range(5) ]".

    These aren't listed separately in the index, where might I find
written discussion of these?
Thanks,
Mike
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