Write a function named `mean`

that takes an argument named `list`

, which is the head cell of a linked list in the form below and returns the mean of the numbers in the data properties. Hint: the most efficient way to proceed is to make one iteration through the list computing the running sum and counting how many numbers are in the sum as you go. Then just divide to get the mean.

```
var tail = { next: null };
var cell2 = { data: 12, next: tail };
var cell1 = { data: 45: next: cell2 };
var head = { next: cell1 };
```

exercise: functions 08