print(d)

==> 1

print(elems[::-1])

The syntax `[x:y:z]` means "take every zth element of a list from index x to index y". When z is negative, it means going backwards. When x isn't specified, it's default to the first element of the list in the direction you traverse the list. When y isn't specified, it's default to the last element of the list. So if we want to take every 2th element of a list, we use `[::2]`.

print(reversed_evens)

==> [0, 2, 4, 6, 8]

We can also use slicing to delete all the even numbers in the list.

print(ngrams(tokens, 3))

==> [['i', 'want', 'to'],

In the above example, we have to store all the n-grams at the same time. If the text has m tokens, then the memory requirement is `O(nm)`, which can be problematic when m is large.

print(ngram)

==> ['i', 'want', 'to']

Another way is generate n-grams is to use slices to create lists: `[0, 1, ..., -n]`, `[1, 2, ..., -n+1]`, ..., `[n-1, n, ..., -1]`, and then `zip` them together.

print(ngram)

==> ('i', 'want', 'to')

Note that to create slices, we use `(tokens[...] for i in range(n))` instead of `[tokens[...] for i in range(n)]`. `[]` is the normal list comprehension that returns a list. `()` returns a generator.