Tricky 27 ("The Island of the Wicker people"), no more than 1 skill per lemming
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This is pretty simple.  Once you get down to the "main island" where the one-way
wall is, you bash to the right, below the one-way wall to bypass it, blocking on 
the left to prevent lemmings from falling.  At some point during the bashing, 
place a blocker ("A") in the bash tunnel such that only 2 lemmings (not counting 
the basher) are to the right of blocker A.  Do this relatively early during the 
bashing, so that the 2 lemmings to the right of blocker A become increasingly 
separated as the bashing tunnel expands.  Once the basher has gone all the way 
through to the right edge of the main island, the 2 lemmings will build to the 
right from the right edge of the island/bash tunnel, and then block off ("B") 
the right end of the bridge.  You can now release the crowd by having a lemming 
from the crowd explode near blocker A to free him.  Use remaining builders to 
build from blocker B up onto the surface of the island, and then up onto the exit.