∀ {b n d fuel nf df : ℕ} {cs : List Char},
  1 < b →
    0 < n →
      d < b →
        b * n + d < fuel →
          n < nf → d < df → b.toDigitsCore nf n (b.toDigitsCore df d cs) = b.toDigitsCore fuel (b * n + d) cs