PrevPrimeInt( n )
PrevPrimeInt
returns the largest prime which is strictly smaller than
the integer n.
Note that PrevPrimeInt
uses a probable-primality test (see
IsPrimeInt).
gap> PrevPrimeInt( 541 ); 523 gap> PrevPrimeInt( 1 ); -2
GAP 3.4.4