First off, what does map(f, xs) represent mathematically in the first place? It should invoke function f(x) for every x in xs. Functions, of course, can take many arguments—single argument functions are just the simplest case. So what would be reasonable to assume map(f, xs, ys) would do? In the blog post, Igor suggests the behaviour should be to chain xs and ys, but chances are they represent completely different things, so chaining them would lead to a heterogenous collection of items. Mathematically, you would expect the function calls made to be f(x1, y1), f(x2, y2), ...