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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(,)
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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.
 
 
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