code: function (note, number) {
    var letter = note.letter;
    var index = scale.indexOf(note.letter);
    var newIndex = mod(index + number - 1, scale.length);
  
    assert(index > -1);
    assert(newIndex > -1 && newIndex < scale.length);
  
    return scale[newIndex];
  }
docs: Increase the letter of a note by a given interval