The problem arises when you are asked to provide further (possibly infinite nr) elements of the sequence: the first would still compress nicely as "a(i)=i*10+1

The problem arises when you are asked to provide further (possibly infinite nr) elements of the sequence: the first would still compress nicely as "a(i)=i*10+1." yours would be "a(3)=pi, a(4)=zebra, a(5)=cheetah, a(6)=akrcqfztaf a(7)=666" etc. But I agree, if the question is just "what is the next element?" and so far only 3 are given, your answer is as good as anything else.