Fixed: split literals were deleted too aggressively from PEL

Suppose we need to split p and q to show c.
Before, the following would happen:
1. p
 2. q
 3. ~q
4. ~p
because during 2./3., q was deleted from PEL.
Now, the following happens:
1. p
 2. q
 3. ~q
4. ~p
 5. q
 6. ~q
7. q
8. ~q
Sadly, this is much more expensive, obviously.