You can refer to several consecutive array elements by an index range:
`x(3:6)` means the four element subarray `[x(3), x(4), x(5),
x(6)]`.

Occasionally, you also want to refer to a sparse subset of an array; you
can add an increment to an index range by means of a second colon:
`x(3:7:2)` means the three element subarray `[x(3), x(5),
x(7)]`.

A negative increment reverses the order of the elements: `x(7:3:-2)`
represents the same three elements, but in the opposite order
`[x(7), x(5), x(3)]`. The second element mentioned in the index
range may not actually be present in the resulting subset, for example,
`x(7:2:-2)` is the same as `x(7:3:-2)`, and `x(3:6:2)`
represents the two element subarray `[x(3), x(5)]`.

Just as the increment defaults to 1 if it is omitted, the start and stop
elements of an index range also have default values, namely the first
and last possible index values. Hence, if `x` is a one-dimensional
array with 10 elements, `x(8:)` is the same as `x(8:10)`. With
a negative increment, the defaults are reversed, so that `x(:8:-1)`
is the same as `x(10:8:-1)`.

A useful special case of the index range default rules is `x(::-1)`,
which represents the array `x` in reverse order.

Beware of a minor subtlety: `x(3:3)` is not the same thing as
`x(3)`. An index range always represents an array of values, while
a scalar index represents a single value. Hence, `x(3:3)` is an
array with a single element, `[x(3)]`.