Ch4, Ex14

This is iterative even without dataflow variables due to the lazy nature and hence incremental computation of successive elements of append result.

fun lazy {LAppend Xs Ys}
    case Xs of nil then Ys
    [] X|Xr then X|{LAppend Xr Ys}
    end
end 

When first element is needed it returns X|<promised next elems> and it calculates the next elements on the need basis.