sub __merge {
   my ($old, $new) = @_;
   my @stack;
   push @stack, { 'ptr' => '', 'old' => $old, 'new' => $new };

   while (@stack) {
      my $el = shift @stack;
      my ($oref, $nref) = (ref($el->{'old'}), ref($el->{'new'}));
      if ($oref eq $nref) {
         unless ($nref) {
            eval "\$old$el->{'ptr'} = \$el->{'new'}";
            $logs->enqueue($@) if $@;
         }
         # we simply assume that there will be no scalar references, and we
         # also tacitly assume all blessed references being hashrefs!
         # for arrays things run rather unspectacularly:
         # iterate over the old list, push the new value on the stack in case it's 
         # a reference, overwrite the old value with that from the new list.
         # if the old list is shorter than the new one perform the same with all overstanding elements.
         elsif ($oref =~ /ARRAY/) {
            my $i;
            for ($i = 0; $i < @{$el->{'old'}}; $i++) {
               if (ref $el->{'new'}->[$i]) {
                  push @stack, { 'ptr' => $el->{'ptr'}."->[$i]", 
                                 'old' => $el->{'old'}->[$i], 
                                 'new' => $el->{'new'}->[$i] 
                               };
               }
               if (exists $el->{'new'}->[$i]) {
                  eval "\$old$el->{'ptr'}"."->[$i] = \$el->{'new'}->[$i]";
                  $logs->enqueue($@) if $@;
               }
               else {
                  splice @{$el->{'old'}}, $i, 1;
                  $i--;
               }
            }
            while (exists $el->{'new'}->[$i]) {
               if (ref $el->{'new'}->[$i]) {
                  push @stack, { 'ptr' => $el->{'ptr'}."->[$i]", 
                                 'old' => $el->{'old'}->[$i], 
                                 'new' => $el->{'new'}->[$i] 
                               };
               }
               eval "\$old$el->{'ptr'}"."->[$i] = \$el->{'new'}->[$i]";
               $logs->enqueue($@) if $@;
               $i++;
            }
         }
         # with hashes things are a bit more complicated, since we know nothing
         # about the keys in each table. thus we have to delete key-value pairs from
         # the old tree in case they don't exist in the new tree. on the other hand, if 
         # a key in the new tree exists that doesn't exist in the old tree it has to be
         # 'installed' there too.
         else {
            my @okeys = sort keys(%{$el->{'old'}});
            my @nkeys = sort keys(%{$el->{'new'}});
            
            while (@okeys) {
               my $key = shift @okeys;
               # we don't now anything about @nkeys, hence we can't rely on the
               # order in which its elements occur:
               if (exists $el->{'new'}->{$key}) {
                  for (my $i = 0; $i < @nkeys; $i++) {
                     if ($nkeys[$i] eq $key) {
                        splice @nkeys, $i, 1;
                        last;
                     }
                  }
               }
               if (ref $el->{'new'}->{$key}) {
                  push @stack, { 'ptr' => $el->{'ptr'}."->{'$key'}", 
                                 'old' => $el->{'old'}->{$key}, 
                                 'new' => $el->{'new'}->{$key} 
                               };
               }
               unless (exists $el->{'new'}->{$key}) {
                  delete $el->{'old'}->{$key};
               }
               else {
                  eval "\$old$el->{'ptr'}"."->{'$key'} = \$el->{'new'}->{'$key'}";
                  $logs->enqueue($@) if $@;
               }
            }
            while (@nkeys) {
               my $key = shift @nkeys;
               if (ref $el->{'new'}->{$key}) {
                  push @stack, { 'ptr' => $el->{'ptr'}."->{'$key'}", 
                                 'old' => undef, 
                                 'new' => $el->{'new'}->{$key} 
                               };
               }
               eval "\$old$el->{'ptr'}"."->{'$key'} = \$el->{'new'}->{'$key'}";
               $logs->enqueue($@) if $@;
            }
         }  
      }
      else {
         eval "\$old$el->{'ptr'} = \$el->{'new'}";
         $logs->enqueue($@) if $@;
      }
   }

   $old;
}