sub add_holiday {
my $self = shift;
my $new_dt = shift;
my @dt = $self->{exception_holidays}->as_list;
my @dt = $self->{single_holidays}->as_list;
push @dt, $new_dt;
$self->{exception_holidays} =
$self->{single_holidays} =
DateTime::Set->from_datetimes( dates => \@dt );
return;
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