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### 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.