code: function sign(extension) {
    var num = extension.number().toString()
      , diff = normal[num] - extension.qualityValue();
  
    if (diff === 1)
      return 'b';
    if (diff === -1)
      return num === '7' ? 'maj' : '#';
    else
      return '';
  }
docs: Return (if necessary) the sign to prepend an extension