sub nestedLoops {
    my( $loops, $params )= @_;
    my $code= $params && $params->{Code};
    my @list;
    my $when= $params && $params->{OnlyWhen}
        ||  sub { @_ == @$loops };
    my $i= -1;
    my @idx;
    my @vals= @$loops;

    my $iter= sub {
        while( 1 ) {
            # Prepare to append one more value:
            if(  $i < $#$loops  ) {
                $idx[++$i]= -1;
                $vals[$i]= $loops->[$i]->(@list)
                    if  'CODE' eq ref $loops->[$i];
            }
            # Increment furthest value, chopping if done there:
            while(  @{$vals[$i]} <= ++$idx[$i]  ) {
                # Return if all done:
                return   if  --$i < 0;
                pop @list;
            }
            $list[$i]= $vals[$i][$idx[$i]];
            if(  ! ref $when  ||  $when->( @list )  ) {
                return @list;
            }
        }
    };
    return $iter   if  ! $code;
    while(  $iter->()   ) {
        $code->( @list );
    }
}