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EXAMPLE
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a(1)=256: there are 256 compositions of even numbers into 9 parts <= 1:
0: (0,0,0,0,0,0,0,0,0) --> 9!/9!0! = 1
2: (0,0,0,0,0,0,0,1,1) --> 9!/7!2! = 36
4: (0,0,0,0,0,1,1,1,1) --> 9!/5!4! = 126
8: (0,0,0,1,1,1,1,1,1) --> 9!/3!6! = 84
10: (0,1,1,1,1,1,1,1,1) --> 9!/1!8! = 9
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