signature MLTON_ARRAY =
   sig
      val unfoldi: int * 'b * (int * 'b -> 'a * 'b) -> 'a array * 'b
   end

• unfoldi (n, b, f)
  constructs an array a of length n, whose elements a_i are determined by the equations b₀ = b and (a_i, b_i+1) = f (i, b_i).