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:)