### 2.3.3 Nil index refers to an entire dimension

When you index a multi-dimensional array, very often you want to let one or more dimensions be "spectators". In Yorick, you accomplish this by leaving the corresponding index blank:

x(3,) x(,5:7) y(,::-1,) x(,) |

In these examples, `x` is a 2-D array, and `y` is a 3-D array.
The first example, `x(3,)`, represents the 1-D array of all the
elements of `x` with first index 3. The second represents a 2-D
array of all of the elements of `x` whose second indices are 5, 6,
or 7. In the third example, `y(,::-1,)` is the `y` array with
the elements in reverse order along its middle index. The fourth
expression, `x(,)`, means the entire 2-D array `x`, unchanged.