\\ a function to iteravely solve for a GF

\\ pr = desired precision of solution, default is x^20
\\ f = the function giving the recursive specification
{
   iterate(f,pr)=
   my(ans,prev,j);
   if(!pr, pr=20);
   ans = 'x + O('x^pr);
   prev = ans;
   ans = f(prev);
   while (ans-prev != O(x^pr),
	  prev=ans;
	  ans = f(prev) + O(x^pr);
	 );
   return(ans);
}