RosettaCodeData/Task/Anonymous-recursion/PARI-GP/anonymous-recursion-1.pari

Fib(n)={
  my(F=(k,f)->if(k<2,k,f(k-1,f)+f(k-2,f)));
  if(n<0,(-1)^(n+1),1)*F(abs(n),F)
};