Here is the question from Rotman, verbatim:
A sequence S'_*\stackrel{f}{\to} S_* \stackrel{g}{\to} S''_* is exact in Comp if and only if S'_{n}*\stackrel{f_n}{\to} S_n \stackrel{g_n}{\to} S''_{n} is exact in Ab for every $n\in \mathbb{Z}$
(Note here Comp and Ab are the categories of chain complexes and abelian groups respectively.
I am slightly confused - I thought this is the definition of an exact sequence of chain complexes. Indeed this is what Massey appears to say (Definition 2.6)
What am I missing here?