sub apply_to {
my ($self, $ss) = @_;
my ($first_alt, @rest_alt) = @{$ss->alternatives};
- return $ss->but(next_step => undef) unless $first_alt;
+ return $ss->but(next_step => $ss->on_exhaustion_step) unless $first_alt;
trace 'search.backtrack.rewind_to' => $first_alt->[1];
return $ss->but(
current_hypothesis => $first_alt->[0],