sbcl: explicitly document the cleanup convention

      sha256 = "189gjqzdz10xh3ybiy4ch1r98bsmkcb4hpnrmggd4y2g5kqnyx4y";

    };

    # The loosely held nixpkgs convention for SBCL is to keep the last two

    # versions.

    # https://github.com/NixOS/nixpkgs/pull/200994#issuecomment-1315042841

    "2.3.2" = {

      sha256 = "sha256-RMwWLPpjMqmojHoSHRkDiCikuk9r/7d+8cexdAfLHqo=";

    };