S->aABA->bBb B->A|epsilonIt seems that the string abbbb can be derived by using more than one ways.After using the starting production and the production for A, we get:S=>abBbB.From here, either of the B's can be replaced with epsilon and the other one replaced with bBb. So, there are two ways of deriving abbbb.Isn't this ambiguous grammar?It's a question from Formal Languages and Automata by Peter Linz (5th edition) Exercise 5.2.16 
It has been asked to prove that a grammar is unambiguous which seem ambiguous to me
context free;formal grammars;ambiguity