Pursuant to the first problem in this series, this one involves checking for divisibility by 9.

Write a function to determine if a number is divisible by 9. This can be done by the following method:

1. Sum the digits of the number. As with methods for other numbers, this summing step can be applied recursively until a single-digit number results. If the resulting number is divisible by nine, then so is the original number.

Previous problem: divisible by 8. Next problem: divisible by 10.