How to understand the following sentences? We can show that the upper bound of Theorem 31.11 is the best possible by showing that the call EUCLID(F+1,F)makes exactly k-1 recursive calls when k≥2. gcd(Fk+1.Fk)=gcd(Fk,Fk+1 mod Fk) gcd(Fk,f-i）.How to understand the following sentences?