drop(0, L) = L
Example: drop(0, [2,4,6]) = [2,4,6].
drop(n, [ ]) = [ ]
Example: drop(5, [ ]) = [ ].
drop(n, L) = drop(n-1, tail(L))
Example: drop(3, [2,4,6,8,10]) = drop(2, [4,6,8,10]) = [8,10].