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## prev_prime
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### Instructions
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Write a **function** which returns the first prime number which is equal or inferior to the `u64` passed as parameter.
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If there are no primes inferior to the `u64` passed as parameter the function should return `0`.
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### Expected function
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```rust
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pub fn prev_prime(nbr: u64) -> u64 {
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}
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```
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### Usage
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Here is a possible program to test your function :
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```rust
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fn main() {
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println!("The previous prime number before 34 is: {}", prev_prime(34));
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}
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```
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And its output :
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```console
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$ cargo run
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The previous prime number before 34 is: 31
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$
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```
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