Test if a value is in a list : contains

### 2.37.24 Test if a value is in a list : `contains`

`contains` takes as argument a list (or a set) `L` and a value `c`.
`contains` tests if `c` is an element of the list `L`.
`contains` returns `0` if `c` is not in `L`, or a strictly positive integer which is 1+the index of the first occurence of `c` in `L`.
Input :

`contains([0,1,2,3,4,2],2)`

Output :

`3`

Input :

`contains(%{0,1,2,3,4,2%},2)`

Output :

`3`