Homology commutes with filtered colimits. In the abelian category
AddCommGrpCat, the degree-n homology functor on chain complexes
preserves filtered colimits. This is the categorical content underlying
Corollary 23.13 (singular homology commutes with direct limits of spaces)
and Lemma 23 (homology of a direct limit of chain complexes is the direct
limit of the homologies).