Function: parsum
Section: programming/parallel
C-Name: parsum
Prototype: V=GGJ
Description:
 (gen,gen,closure):gen parsum($1, $2, $3)
Help: parsum(i=a,b,expr): the sum (i goes from a to b) of
 expression expr, evaluated in parallel (in random order).
Doc: sum of expression \var{expr}, the formal parameter
 going from $a$ to $b$, evaluated in parallel in random order.
 The expression \kbd{expr} must not access global variables or
 variables declared with \kbd{local()}, and must be free of side effects.
 \bprog
 ? parsum(i=1,1000,ispseudoprime(2^prime(i)-1))
 cpu time = 1min, 26,776 ms, real time = 5,854 ms.
 %1 = 20
 @eprog
 returns the number of prime numbers among the first $1000$ Mersenne numbers.
 %\syn{NO}
