code: function decode3(s) {
    assert.equal(s.length, 3)
    const [ upper, mid, lower ] = s
    return (decode(upper) << 12) | (decode(mid) << 6) | decode(lower)
  }
docs: Decodes three chars into an 18 bit number
  
  @name decode3
  
  @param {String} s the chars to decode
  @returns {Number} an 18 bit number