Write a function to determine if a number is divisible by 13. Similar to the number seven, this can be done by a few different methods:
- Form the alternating sum of blocks of three (separated at the comma locations). Apply this recursively until a three-digit number remains. If this number is divisible by 13, then so is the original number. This three-digit number can also be further reduced using one of the other methods below.
- Add four times the last digit to the remaining number. Apply recursively until a two-digit number remains. As before, if this number is divisible by 13, then so is the original number.
- Similar to the previous method, multiply the last digit by nine and subtract it from the remaining number. Apply recursion, etc.
Some of the function restrictions have been lifted.
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