class Tap {}

my $s = Supplier.new;
my $tap = $s.Supply.on-close({ say "Tap closed" }).tap(
    -> $v { say "the value is $v" },
    done    => { say "Supply is done" },
    quit    => -> $ex { say "Supply finished with error $ex" },
);

# later
$tap.close;

method close(Tap:D:)