Ironically within the level text files there is the tile modifier 'f', which means: Tile is mirrored vertically. The flip horizontal button is implemented in the text files via combination of 'f' and 'r' ('r'=rotate 90 degrees clockwise), e.g. 'frr' for flipping horizontally a non-rotated tile.

Which tags are currently in use? I ask since tagged levels can already be found via search.

I don't believe any replays exist on the github for the outtake levels of the community pack like "bulldozer"? unless I'm wrong. Does anybody have replays for those levels?

The replays for the outtake levels that are shipped with Lix (including Bulldozer) are in the github repo: https://github.com/SimonN/lemforum-replays/tree/master/misc/lemforum-outtakes

Only verified for Arch-Linux, but very well possible that it affects Win as well:

• Cutting edge DMD v2.098.1 doesn't build a release build anymore:
  Error:
  'function  file.filename.base.IFilename.isChildOf
  FAIL .dub/build/application-release-linux.posix-x86_64-dmd_v2.098.1-B60974BF1C37AB9E479B0D5164D1DB7B/ lix executable
  dmd failed with exit code -4.
  Full exception: object.Exception@source/dub/compilers/compiler.d(116): dmd failed with exit code -4.
  ----------------
  ??:? [0x55bc83f0c226]'
  
  
• Cutting edge ldc 1.28.1 doesn't build anymore without increasing the stack size (on Linux possible with e.g. 'ulimit -s 16384')
  Error could be related to [Windows] ldc2 fails to compile project with various sudden halts seen in -vv output #3913 (stack overflow)

Introduce physics changes only when well-tested in singleplayer, and move all culture to the new stable. Ideally, create some unittests for physics.
Only extend the level format, don't replace it entirely.

I think breaking the level format should only be done if absolutely needed. Backwards compatibility has a high value in this respect since Lix has a lot of levels. Physics changes don't need to necessarily lead to much breakage.

I agree with WillLem that an experimental branch would be a good idea even in the careful approach. To have physics changes well-tested you need them tested in some way, which can be achieved with an experimental branch. An experimental branch would give the courage to pull through the physics changes, which are in the waiting line for so long.

The levels in the Superlemminas pack are to the most part not puzzle oriented, so for me it was less about getting my route/solution denied. The traps didn't change the route but rather needed just one stair (and maybe some more for safety)/bash to get around them. Only in one level it made the skills more tight, else the skills were in such levels plenty enough to not be a ressource problem. Memorizing was actually no problem either since they are in obvious places if you know it.

But I don't think the levels are really explorative either, they are for my skill level brutally execution demanding. Not sure if every platformer player likes hidden traps.

I can see that it is a matter of taste, some people even like troll levels, some like the explorative aspect of hidden objects like WillLem.
But my personal problems with the hidden traps in the Superlemminas pack were:

1) The challenge wants the player to abstain from using tools the engine actually grants them by itself. This makes the levels much harder. And then when one thinks the level is beaten, a hidden trap crushes the success just before the exit. The player can feel trolled or cheated by that, especially since the challenge asked them to play fair and clear by the rules. This could be an incentive to just use action replay. After it happened to me I gathered myself and started freshly over, kept the rules. But it was really harsh for me.

2) I actually didn't expect the deliberate use of hidden traps in such a challenge, which made them hit harder as Proxima indicated. On the other hand one loses trust in the levels and fears traps everywhere.

3) Generally in hard/at the end of levels they have a bigger impact than at the beginning/in easier ones.

I'm actually not so into memorizing things as well though as I said that was not the problem here. I took the challenge because I wanted to improve my skills executionwise.

Thanks for fixing.