List Type: inconsistency in error feedback

[dmonad]

@cphyc noticed the following inconsistency:
And their may be some inconsistency in the List.val function, when you try to retrieve a negative index, it throws an error Error: this position does not exist
But when you try to get any index > 0, their's no error even though the index is undefined (then you get the last index of your array)

Note to me:
I've to check every "public" method for valid input parameters. I'll close this when I did..