+ };
+ my @bytime = @p;
+ my $order = $get_t->($bytime[0]) <=> $get_t->($bytime[1]);
+ if ($order > 0) { # newer first
+ } elsif ($order < 0) {
+ @bytime = reverse @bytime;
+ } else {
+ return $unknown->('merge of two identical same-age parents');
+ }