i managed to do the one group so i'll do some testing with that soon, but i also wanna give a detailed explenatin of why the 19 skill zip worked and why the 18 skill one didnt.
To do the zip you use a 3px ledge, this means that the max width of the lemming group is 1px however in certain circumstances you can do it with a 2px group width. to do this the update order within a Q must match up corectly, the builder (B) and first pixel of the lemming group (GP1) and second pixel of the lemming group (GP2) must be in a percise update order within the Q's. this is what has to happen within the 2Q's of the wallzip, Q1: B will go into the position 1Q before placeing the brick down, GP1 starts to jump, Q2: GP2 starts jumping, B places brick, GP1 continues jumping due to being updated after B was. if this doesnt happen exactly like that in the exact same order, the 2px width zip wont work, your group in both S19 and S18 are 2px groups, the reason why S19 works is because the lemmings are in that exact update order within Q's, in S18 to get the lemmings into one group rather than 1 group and 1 seperate lemming it looks like you messed up the exact order in which the lemmings have to be updated. im pretty confident that S18 is doable, maybe with a diffrent zip setup but thats something i'll find out when im back working on it.
