P02: tailrec added
P02: that returns nth element
P02: that fails if nth bigger then size of the list
P02: that fails if nth smaller then 0
P03 - implementation change not to use built-in api
P03 - add @tailrec, also case matching given integer value (x < 0)
P04 - that empty list is of length 0
P04 - that one-element list is of length 1
P04 - that returns the list length