It sounds like you are a commander keen fan Namida. I've never played any of them before but I plan to one day.

Commander Keen: Goodbye Galaxy, specifically Episode IV: Secret of the Oracle (the shareware episode) is one of my favourite games of all time. It's very much designed as a children's game, but it is legitimately difficult on Hard, and is pretty darn enjoyable if you ask me.

Never found the level editors for it though. I did find a level editor for the original Duke Nukem platformer, but that series doesn't have the same finesse as Commander Keen.

Ah, okay then.

If, when playing a level in the NeoLemmix player, pressing Alt+F4 to quickly quit out causes an untrapped EAccessViolation exception to be raised. While not major because you desire to exit anyway, it's a little unclean and causes a "Stopped working" message to appear.

Well, I wouldn't say duplicated, but every line of code that controls vertical movement would need to be replaced with a conditional branch (e.g. changing "Inc(Y);" to "if bInverted then Dec(Y) else Inc(Y);", or having a "DeltaY" value that's added to the coordinates but which is negated upon inversion etc). And yes, I propose all skills be assignable to inverted lemmings (I'll adjust my first post).

As namida said though... if antigravity were implemented, then an assignable tool would be an easy enough addition. Having an interactive object that changes gravity (Cheapo Copycat had 4 types... "always animated: set lemming to ceiling", "always animated: set lemming to floor", "animate on activation: set all lemmings to ceiling", "animate on activation: set all lemmings to floor") would depend on the tileset and what puzzle is desired. The difference between the tool and an interactive object is that the tool would toggle the lemming's orientation rather than explicitly setting it to the ceiling or the floor, so has a bit more flexibility and puzzle application.

Here's a second attempt, although I'm worried it looks maybe a bit too comical!

So I confess this is partially based off the old "Cheapo Copycat" mechanics, but I would like to propose a new tool that, if used on a lemming, causes him to flip upside-down and be pulled towards the ceiling (or the right way up again if the lemming was already upside-down).

When upside-down, all subsequent tools perform as expected, but with the up and down directions flipped, so using a Miner will cause the lemming to move towards the top of the level, for example.

When used, the lemming will likely fall in his new direction, and might splat if not careful.

Nuances:

• The lemming inverts about his feet. This ensures the lemming doesn't become stuck in the terrain upon inversion
• A lemming walking on the ceiling can have all skills assigned to it as normal - this includes the Cloner, which will create another lemming walking upside-down, and the Inverter itself which will switch the lemming back to the floor
• Can be used on a lemming currently performing a task, which may cause their vertical direction to change in the case of Builders, for example, but will likely cause them to stop if their feet are in open air after inversion
• If used on a Swimmer, the lemming will (normally) fall out of the water
• Can be used on a Blocker, but will remain a Blocker if his feet is on terrain after inversion (which will happen if the Blocker is inside a tunnel that's only a pixel tall)
• If used on a Climber, the lemming will invert but continue to climb the wall he's climbing (albeit in the other direction)
• Splat distance is NOT reset if used on a Faller
• An inverted lemming interacts with obstructions and objects as normal
• Cannot be used on an Ohnoer (Bomber), Exiter, Flipper, Splatter, Frier/Vaporizer, Drowner or Stoner, or a lemming dying to a custom trap (because the lemming object no longer exists).
• The Inverter is not a permanent ability - if used on a Walker, the lemming afterwards will remain a Walker, just with gravity switched

The icon, I suggest, should be of a Lemming that is facing right but falling forwards, with an up arrow to his upper left, and a down arrow to his lower right.

How's it sound?

Möbius gave me the working copy of the Revenge of the Lemmings files last year, so I should still have them on my systems somewhere - I'll see if I can dig them out.  Granted they were a work in progress.