backward member implementation question

Why can't [this function](https://github.com/karpathy/micrograd/blob/master/micrograd/engine.py#L54) simply be implemented as follows? Am I missing something? We are dealing with a composite structure.

```
def backward(self, is_first=True):
if (is_first == True):
self.grad = 1.0

self._backward()

for c in self._prev:
c.backward(False)
```