Simple References $p[n] 
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A simple reference allows you to access one specific value from the parameter.
$p=list("red","orange","yellow","green","blue","indigo","violet")
In this example, if you use $p in your equation, it will be replaced with a random color from the set. If you wanted to specifically use blue, you can reference it with $p[5].
One example usage for references might be with arithmetic progressions.
$a=range(4,1000,3)
This range parameter will select from the values:
4, 7, 10, 13, 16 ... 994, 997, 1000
These numbers are in arithmetic progression.
$a[14] will give you the 14th number in the arithmetic progression (43). In general, $a[n] will give you the nth number in the arithmetic progression.
