Function: primes
Section: number_theoretical
C-Name: primes
Prototype: L
Help: primes(x): returns the vector of the first x primes.
Doc: creates a row vector whose components
 are the first $x$ prime numbers, which must be among the precalculated primes.
 \bprog
 ? primes(10)           \\ the first 10 primes
 %1 = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
 ? primes(primepi(10))  \\ the primes up to 10
 %2 = [2, 3, 5, 7]
 @eprog
