[x1, ..., xm] ++ [y1, ..., yn] == [x1, ..., xm, y1, ..., yn] [x1, ..., xm] ++ [y1, ...] == [x1, ..., xm, y1, ...]If the first list is not finite, the result is the first list. WARNING: This function takes linear time in the number of elements of the first list.