After this, just one more skill to go!
 
The code omitted definition for lemming.ApplyBashingMask().  The mask graphics in DOS Lemmings have the same dimensions (16x10) as the animation frames for bashing, and there are 4 masks, with left/right versions.
 
So:
 
1) upper-left of the 16x10 mask graphics should be located at (lemming.x + lemming.frameLeftdx, lemming.y + lemming.frameTopdy)
 
2) if we index the 4 masks as 0 - 3, use the mask indexed lemming.animationFrameIndex - 2
 
3) don't forget to check lemming.dx to pick the left/right version