sub count {
	my ($data, $fields) = @_;
	$fields = [$fields] unless ref $fields;
	my $count = {};
	for my $obj (@$data) {
		my $ref = $count;
		$ref = $ref->{ $obj->{ $fields->[$_] } } //= {}
			for 0..$#$fields-1;
		$ref->{ $obj->{ $fields->[-1] } }++;
	}
	return $count;
}