### Difference between f and f(r)

I was experimenting with symbolic functions, and I was wondering what is the difference between these two forms of creating one:

```
sage: f1 = function('f', r)
sage: f2(r) = function('f', r)
```

I have made some experiments in this Sage worksheet, but except for some output differences, I don't know what will happen if I try to differentiate them, integrate them, put them into an equation, etc.

Any ideas?