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  sub combine_factors {
    my $list = shift;
...
  ($fa->{$_} ||= 0) -= $fb->{$_} for keys %$fb;
  my @c = map +($_) x $fa->{$_}, grep $fa->{$_} > 0, keys %$fa;
  my @d = map +($_) x -$fa->{$_}, grep $fa->{$_} < 0, keys %$fa;