Fibonacci

[Emil Jerabek]

This is not a bug report, just a suggestion. 

A comment in mpz/fib_ui.c says something about computing the Fibonacci
numbers using Lucas numbers, since that would enable to replace 
general multiplication by squaring in the recursive formula.

There is a simple way how to achieve this without using the Lucas numbers as
an auxiliary sequence:

F[2n]   = 2*F[n+1]^2 - 3*F[n]^2 - (-1)^n
F[2n+1] =   F[n+1]^2 +   F[n]^2


                                                       Emil Jerabek