% Generates the primes up to n (not inclusive).
  It works using a primes sieve algorithm, by first generating the primes
  up to sqrt(n) and then using those to sieve the values up to n.
  A full description of this algorithm is given in the Nesl manual. %
function primes(n) =
if n == 2 then [] int
else 
  let sqr_primes = primes(ceil(sqrt(float(n))));
      sieves = {[2*p:n:p]: p in sqr_primes};
      flat_sieves = flatten(sieves);
      flags  = dist(t,n) <- {(i,f): i in flat_sieves};
  in drop({i in [0:n]; flags | flags}, 2) ;