rand(v) {a 
a :: (a in number)}
For a positive value v, rand returns a random value
in the range [0..v). Note that the random number seed is reset each
time the user returns to top level. To get different sets of random numbers,
use rand_seed with different seeds.
rand_seed(v) {int 
bool}
Seed the random number generator. Note that a given seed is only guaranteed
to give the same sequence of random numbers on a fixed machine and with
a fixed number of processors.