code: function decode2(s) {
    assert.equal(s.length, 2)
    const [ upper, lower ] = s
    return (decode(upper) << 6) | decode(lower)
  }
docs: Decodes two chars into a 12 bit number
  
  @name decode2
  
  @param {String} s the chars to decode
  @returns {Number} a 12 bit number