An 'emirp' (prime spelled backward) is a prime whose digits when reversed, yields another prime. For example, 31 is an emirp of 13 and vice-versa. The definition of an emirp does not include palindromic primes such as 101 as they yield the same number when their digits are reversed.

Consider the unordered pair \((p_1 , p_2)\) where \(p_1\) and \(p_2\) are primes which are emirps to each other. How many such unordered pairs of emirps exist which are less than 1000?

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