sub up_modified_bis{
	my $ind = ' ' x $#_ x 4;
	print  $ind."\@_ is ";
	dd @_;
	my ($row, $col) = ($_[0][0],$_[0][1]);
	print $ind."receiving row $row col $col \n";
	
	if ($row == 0 and $col == 0){
		print  $ind."RETURNING: ";
		dd @_;
		return @_;
	}
	else{
		(
			$row * $col > 0 && up_modified_bis( [~-$row, ~-$col],map {[@$_]}@_ ) 
		).
		(
			$row > $col && up_modified_bis( [~-$row, $col], map {[@$_]}@_ )
		);
	}	
}


#output of called; up_modified_bis ([(3,1)]);

@_ is [3, 1]
receiving row 3 col 1
    @_ is ([2, 0], [3, 1])
    receiving row 2 col 0
        @_ is ([1, 0], [2, 0], [3, 1])
        receiving row 1 col 0
            @_ is ([0, 0], [1, 0], [2, 0], [3, 1])
            receiving row 0 col 0
            RETURNING: ([0, 0], [1, 0], [2, 0], [3, 1])
    @_ is ([2, 1], [3, 1])
    receiving row 2 col 1
        @_ is ([1, 0], [2, 1], [3, 1])
        receiving row 1 col 0
            @_ is ([0, 0], [1, 0], [2, 1], [3, 1])
            receiving row 0 col 0
            RETURNING: ([0, 0], [1, 0], [2, 1], [3, 1])
        @_ is ([1, 1], [2, 1], [3, 1])
        receiving row 1 col 1
            @_ is ([0, 0], [1, 1], [2, 1], [3, 1])
            receiving row 0 col 0
            RETURNING: ([0, 0], [1, 1], [2, 1], [3, 1])