New puzzle

[notaferrariowner]

def sat(x: str): 
    """optional problem description"""
    return "Hello " + x == "Hello world"   # change this to your puzzle

Solvers, post your solutions in the comments using the following formatting:

<details><summary>Reveal solution</summary>

```python
def sol():
    return "world"  # replace with your solution
```
</details>