Taxing 3, no more than 1 lemming assigned skills
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The solution is relatively straightforward, the only trickiness with this level
is that you must not waste time doing unnecessary things.  My solution winds up
with just 3 seconds left on the clock in DOS Lemmings.

The screenshots explain much of the solution.  You start off digging down 10 
pixels and then build.  After the builder turns around, there are only 3 
lemmings in the pit, with the former builder behind the other 2 walker lemmings.  
So it's relatively easy to assign him climber right after he turns around 
heading right.  Then you dig down 9 pixels before building towards the exit, 
stretching your builders fully.  This will allow the builder to bump into the 
edge of the exit platform, so he can turn around, walk back down, and bash to 
free the crowd.