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400. Prolog (SWI), 314 bytes, A000279
Thanks @user for helping me debug my code in the Prolog chat room!
factorial(0,1).
factorial(N,X):-
A is N-1,
factorial(A,B),
X is B*N.
choose(N,R,X):-
K is N-R,
factorial(N,A),
factorial(R,B),
factorial(K,C),
X is A/(B*C).
a(_,0,0).
a(N,K,X):-
A i...