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## doubtful
### Instructions
Create a function named `doubtful` which appends a question mark to every string passed to it. It must not return a value.
```rust
pub fn doubtful(s: /*give the correct type*/ ) {
}
```
You'll need to complete the function signature, so that it works properly with the usage example. You'll also need to complete the usage if you plan to use it.
### Usage
Here is a program to test your function
```rust
fn main() {
let mut s = String::from("Hello");
println!("Before changing the string: {}", s);
doubtful(/*add your code here*/);
println!("After changing the string: {}", s);
}
```
And its output
```console
$ cargo run
Before changing the string: Hello
After changing the string: Hello?
$
```
