sub _build_net_income {
  my $self  = shift;
  my $gross = $self->gross_income;
  my $income_tax = do {
       if ($gross <   6_475) { 0 }
    elsif ($gross <  43_875) { (0.2 * ($gross - 6_475)) }
    elsif ($gross < 150_000) { (0.2 * (43_875 - 6_475))
                             + (0.4 * ($gross - 43_875)) }
    else                     { (0.2 * (43_875 - 6_475))
                             + (0.4 * (150_000 - 43_875))
                             + (0.5 * ($gross - 150_000)) }
    };
  my $child_tax_credit = 545 * scalar @{$self->children};
  my $taxes = $income_tax - $child_tax_credit;
  $taxes = 0 if $taxes < 0;
  return $gross - $taxes;
}