Function: precprime
Section: number_theoretical
C-Name: precprime
Prototype: G
Help: precprime(x): largest pseudoprime <= x, 0 if x<=1.
Description:
 (gen):int        precprime($1)
Doc: finds the largest pseudoprime (see
 \tet{ispseudoprime}) less than or equal to $x$. $x$ can be of any real type.
 Returns 0 if $x\le1$. Note that if $x$ is a prime, this function returns $x$
 and not the largest prime strictly smaller than $x$. To rigorously prove that
 the result is prime, use \kbd{isprime}.
