  We make explicit some conditions on a semi-abelian category D such that, for
any abelian group A in D and any object Y in D, the cohomology group
homomorphisms with coefficients in A, induced by the inclusion of the abelian
objects of D at the level of the slice category D/Y, are actually isomorphisms.
These conditions hold in particular when D is the category Gp of groups, and
this allows us to give a new insight on the Eilenberg-Mac Lane cohomology of
groups. They hold also when D is the category K-Lie of Lie-algebras.
