Interesting problem for testing
How is maximal number of positions?
Only 3 black pieces = 643 (in worst case, include illegal positions) and 10 white pieces with 2 states (captured or not) = 210=1024. Maximum number of all positions is < 268 435 456. With 14 pieces (if 1 byte for 1 piece) need less than 3.5GB memory.
Fortunately, really total number of positions in 21 moves is only 46722936 and necessary memory ~ 650 MB. Value of this memory size is very important!
FIDE Album 1986-88, G056
First I tested this problem using Alybadix 2001 under Windows98 with 550MB RAM and interrupted after 6 hours. After first "pruning" of hash memory solving continued very slowly, probably more than 100 hours...
But with 830 MB RAM problem testable (without pruning!) and after 9 hours and 6 minutes great result: C+ !
Vaclav Kotesovec, tested 28.3.2001