#!/usr/bin/perl
use strict;
use warnings;

sub sum1 {
	for my $tot (shift) {
		my $n_undef = 0;
		for (@_) { ++$n_undef if !defined $_ }
		if (!defined $tot && $n_undef == 0) {
			$tot = 0;
			for (@_) { $tot += $_ }
			return 1;
		} elsif (defined $tot && $n_undef == 1) {
			my $x = $tot;
			my $ref;
			for (@_) {
				if (defined $_) { $x -= $_ }
				else { $ref = \$_ }
			}
			$$ref = $x;
			return 1;
		} elsif (defined $tot && $n_undef == 0) {
			my $tot2 = 0;
			for (@_) { $tot2 += $_ }
			return $tot == $tot2;
		} else {
			return 0;
		}
	}
}

sub add {
	my $tot = 0;
	for (@_) { $tot += $_ }
	return $tot;
}

sub subt {
	my $diff = shift;
	for (@_) { $diff -= $_ }
	return $diff;
}

sub sum {
	my $ref = find_result(@_);
	if (!defined $_[0]) {
		$$ref = add(find_args(@_));
	} else {
		$$ref = subt(find_args(@_));
	}
}

sub find_result {
	return \grep {!defined $_} @_;
}

sub find_args {
	return grep {defined $_} @_;
}



sub prod {
}


my ($a, $b, $c, $d) = (1, undef, 2, 3);

sum($a, $b, $c, $d);

print "$a $b $c $d\n";



# my $ref = find_result($a, $b, $c, $d);
# $$ref = 6;
# 
# my @args = find_args($a, $b, $c, $d);
# $args[0] = 7;
# 
# print "@args\n";
# print "$a $b $c $d\n";


# print "$b\n";

#my (@foo) = (1, 2, 3, 4);