parallel pari: parfirst

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

Hello PARI developers,

I am experimenting with a new parallel primitive which is a kind of
parapply with early return:

?? parfirst(f, x, {&b}):

   Return the smallest index i such that f(x[i]) is non zero, or 0 if none.  If
present, b is to f(x[i]).

One example program:

inline(chkell);
chkell(p)=a->my(E=ellinit([1,a],p));isprime(ellcard(E));
ellp(p)=parfirst(chkell(p),[1..10000]);

ellp(p) returns the smallest a>=1 such that the curve y^2=x^3+x+a has prime
order over F_p.

Suggestions for the name and/or the interface ?

The requirement of an explicit vector for x is a bit burdensome, since 
we might like x to be unbounded (i.e. infinite).

Cheers,
Bill.