TASK #1 - Missing Permutation

You are given possible permutations of the string 'PERL'.

PELR, PREL, PERL, PRLE, PLER, PLRE, EPRL, EPLR, ERPL,
ERLP, ELPR, ELRP, RPEL, RPLE, REPL, RELP, RLPE, RLEP,
LPER, LPRE, LEPR, LRPE, LREP

Write a script to find any permutations missing from the list.

MY NOTES: should be easy, find all permutations and set subtract.
Reuse my Perms module from challenge 149.


TASK #2 - Padovan Prime

A Padovan Prime is a Padovan Number that's also prime.

In number theory, the Padovan sequence is the sequence of integers P(n)
defined by the initial values.

P(0) = P(1) = P(2) = 1

and then followed by

P(n) = P(n-2) + P(n-3)

First few Padovan Numbers are as below:

1, 1, 1, 2, 2, 3, 4, 5, 7, 9, 12, 16, 21, 28, 37, ...

Write a script to compute first 10 distinct Padovan Primes.
Expected Output

2, 3, 5, 7, 37, 151, 3329, 23833, 13091204281, 3093215881333057

MY NOTES: ok, Padovan numbers are rather like Fibonacci numbers,
and easy enought to calculate.  Then we must check isprime().
Should be pretty easy in principle, but in practice I note how big
answers get very quickly, this code finds the first 8 Padovan
Primes but would take ludicrously long amounts of time - and ludicrously
large amounts of RAM to store all the prime numbers.  It's never
finished for N==9 or 10.