Checking for a permutation: is_permu

### 5.9.7 Checking for a permutation: is_permu

A permutation can be written as a list, but not every list corresponds to a permutation. The is_permu is a boolean function which checks to see if a given list is a permutation.

- is_permu takes one argument:
  L, a list.
- is_permu(L) returns 1 if L is a permutation and returns 0 if L is not a permutation.

#### Examples

- Input:
  is_permu([2,1,3])
  Output:

  0

- Input:
  is_permu([2,1,3,0])
  Output:

  1