[Retros] Add units, 20th update

[Alain BROBECKER]

Dear retro friends,

As usual:
http://abrobecker.free.fr/chess/addunits.pdf

Problems modified are 2A, 1KA, 1QA, 1BA, 1NA
Problem added is 6A
Problem removed is N1QAi

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.

Best regards,
Alain