Function: parsum
Section: programming/parallel
C-Name: parsum
Prototype: V=GGJDG
Description:
 (gen,gen,closure,?gen):gen parsum($1, $2, $3, $4)
Help: parsum(i=a,b,expr,{x}): x plus the sum (X goes from a to b) of
 expression expr, evaluated in parallel (in random order).
Doc: sum of expression \var{expr}, initialized at $x$, 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))
 @eprog
 returns the numbers of prime numbers among the first $1000$ Mersenne numbers.
 %\syn{NO}
