Write a function named `variance`

that takes an argument named `list`

, which is the head cell of a linked list in the form below and returns the variance of the numbers in the data properties. Hint: the best way to proceed is to keep a running sum of the numbers and also of the numbers squared, as well as keeping track of how many numbers are in the sum. Then compute the mean of the numbers squared as well as the mean of the numbers. Square the simple mean and subtract from the mean of the squares to get the answer.

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

exercise: functions 09