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find the previous prime number
PrevPrime(N: INT): INT
PrevProbPrime(N: INT): INT 
The first function computes the largest prime number smaller than
N
.
If
N
is negative or too large then an error is signalled.
The upper limit depends on the computer you are using;
it is probably
2^31 or
2^63.
The second function computes the largest probable prime number greater than
N
.
To be absolutely certain the number produced is prime,
you must call
IsPrime
on it, but this may be very costly.
/**/ PrevPrime(1000);
997
/**/ PrevProbPrime(10^50);
99999999999999999999999999999999999999999999999943
