# start off in write-only mode
sub new {
open(my $io, '+>', undef) or die "open: $!";
+ # latest_cmt is still useful when the newest revision is a `d'(elete),
+ # otherwise we favor $sync->{latest_cmt} for checkpoints and {quit}
bless { wr => $io, latest_cmt => $_[1] }, __PACKAGE__
}