[Retros] Add units, 20th update

[Mu-Tsun Tsai]

> Michel also gave 3A, 4A, 5A problems but i'm not sure i'll include them.

> Maybe it's best to keep only "add 1", "add 2" and "add max" problems.

> Note that 5A & 6A (or 9C and 12C) are not in the same category since 5A 

> has

> no promoted piece.


Personally I don't see any reason not to include everything. Certainly 
people will be interested in "how many units is needed for forcing a 
particular number of units to be added" that kind of thing. So I'll 
encourage you to keep tracking the records for all the entries in the table.

Mu-Tsun