my (@prev_pop, @curr_pop);
while (@$prev_subj && @$subj && $subj->[-1] eq $prev_subj->[-1]) {
push(@prev_pop, pop(@$prev_subj));
push(@curr_pop, pop(@$subj));
my (@prev_pop, @curr_pop);
while (@$prev_subj && @$subj && $subj->[-1] eq $prev_subj->[-1]) {
push(@prev_pop, pop(@$prev_subj));
push(@curr_pop, pop(@$subj));