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## delete_prefix
### Instructions
Define the function `delete_prefix(prefix, s)` which returns the string slice `s` with the `prefix` removed wrapped in `Some` . If `prefix ` is not a prefix of `s` it returns `None` .
### Expected Function
```rust
pub fn delete_prefix(prefix: & str, s: & str) -> Option< & str> {
}
```
### Usage
Here is a program to test your function.
```rust
use delete_prefix::delete_prefix;
fn main() {
println!("{:?}", delete_prefix("ab", "abcdefghijklmnop"));
println!("{:?}", delete_prefix("x", "abcdefghijklmnop"));
}
```
And its output:
```console
student@ubuntu:~/delete_prefix/test$ cargo run
Some("cdefghijklmnop")
None
student@ubuntu:~/delete_prefix/test$
```