A *self-describing number* is a
number of which the digits in every position indicate the amount of times
they occur in the number. Here, the positions of the digits are numbered
from left to right starting with 0. The number 2020, for example, is a
self-describing number of four digits. Position "0" has 2 as a value and
there are a total of 2 zeroes in the number. Position 1 has 0 as a value,
1 doesn't occur in the number. Position 2 has value 2 and the digit occurs
twice in the number. Lastly, position 3 has 0 as a value. 3 does not occur
in 2020.

Write a function `selfdescribing`
to which a number $$n \in \mathbb{N}$$ must be given as an argument. The
function must print a Boolean value as a result, indicating whether the
number is self-describing or not.

```
>>> selfdescribing(1210)
True
>>> selfdescribing(1110)
False
>>> selfdescribing(2020)
True
>>> selfdescribing(21200)
True
```