Write a definition of function removeFirst(n, s),
where n is a nonnegative integer and s is a string.
It should yield the string that you get be removing the first
n characters from string s. (If s has
fewer than n characters, then removeFirst(n, s)
should yield an empty string.)
removeFirst(0, "rabbit") = "rabbit"
removeFirst(1, "rabbit") = "abbit"
removeFirst(2, "rabbit") = "bbit"
removeFirst(3, "rabbit") = "bit"
removeFirst(4, "rabbit") = "it"
removeFirst(5, "rabbit") = "t"
removeFirst(6, "rabbit") = ""
removeFirst(7, "rabbit") = ""
removeFirst(2, "abcd") = "cd"
removeFirst(5, "") = ""