(map,ChainComplex,ChainComplex,Function)

map(C,D,f) -- construct a map from the chain complex D to the chain complex C which in degree k is the map provided as the value of f(k).

As a convenience, the value returned by f(k) is used as the third argument in map(C_k,D_k,f(k)).

The function f is called only for those indices which represent spots occupied in both the source and target chain complexes.