code: function getAbcKey(fifths, mode) {
    if (typeof mode === 'undefined' || mode === null) mode = 'major';
    return circleOfFifths[mode][fifths];
  }
docs: Returns the key for abc notation from given fifths
  @param {number} fifths - The position inside the circle of fifths
  @param {string|undefined} mode - The mode (major / minor)
  @returns {string}