code: function (letter1, letter2) {
    var index1 = scale.indexOf(letter1);
    var index2 = scale.indexOf(letter2);
    var distance = mod(index2 - index1, scale.length) + 1;
  
    assert(index1 > -1);
    assert(index2 > -1);
    assert(distance > 0 && distance <= scale.length);
  
    return distance;
  }
docs: Find the interval number between two notes given their letters