If I understand correctly not applicable here (patent expired after max duration of 20 years), but one facet of this is that in some cases you can buy a patent

If I understand correctly not applicable here (patent expired after max duration of 20 years), but one facet of this is that in some cases you can buy a patent that is expired because the owner decided it wasn't worth to continue pay the extension fees and un-expire it by paying the fees retroactively.