$p[0] = The Number of Possibilities

When using references, the available possibilities are referenced from $p[1] through to the last available reference.  There is no $p[0] value. Instead, the 0th reference is reserved for the number of possibilities available in the list, range, coprimes, shuffle or deck.

For example, if you create the parameter.

$p=coprimes(2,10)

The parameter might be loaded with {6,5,7}.

You can use these values individually using references.

$p[1] will be replaced by 6, $p[2] will be replaced by 5 and $p[3] will be replaced by 7

If you use $p[0] in your equation it will be replaced by 3 - the number of coprimes in the set.