A year is leap when it is divisible by 4 and is not divisible by 100 except it is also divisable by 400. Some examples:
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1900 is divisible by 4 is divisible by 100 is NOT divisible by 400 => is NOT leap
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1996 is divisible by 4 is NOT divisible by 100 => IS leap
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2000 is divisible by 4 is divisible by 100 is divisible by 400 => IS leap
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2001 it NOT divisible by 4 => is NOT leap