Works like utils::head() and utils::tail().
# S3 method for index0
head(x, ...)# S3 method for index0
tail(x, ...)
An index0 object
Other arguments, passed to generic function
An index0 object
Just because an object is zero-indexed, doesn't mean that the definition of,
for example, "the first 5 elements" or "the last two elements" has changed.
Thus we add methods head() and tail() to ensure they behave as normal.