code: function encode2(n) {
    assert(0 <= n && n <= 0xfff, ENCODE_OUTOFBOUNDS + n)
    // 12 bits -> 2 digits
    const lower = isolate(n, 0, 6)
    const upper = isolate(n, 6, 6)
    return encode(upper) + encode(lower)
  }
docs: Encodes a 12 bit number into two URL safe chars
  
  @name encode2
  
  @param {Number} n a 12 bit number
  @returns {String} the chars