code: function encode3(n) {
    assert(0 <= n && n <= 0x3ffff, ENCODE_OUTOFBOUNDS + n)
    // 18 bits -> 3 digits
    const lower = isolate(n, 0, 6)
    const mid = isolate(n, 6, 6)
    const upper = isolate(n, 12, 6)
    return encode(upper) + encode(mid) + encode(lower)
  }
docs: Encodes a 18 bit number into three URL safe chars
  
  @name encode3
  
  @param {Number} n a 18 bit number
  @returns {String} the chars