/**
     * Returns a string uniquely defining this music.
     *
     * Each note will be represented according to its ABC notation representation including its letter, any accidentals,
     * and octave (see Pitch.toString). This will be immediately followed by its duration (Double string representation)
     * Each rest will be represented by the letter 'z' followed by its duration in Double representation
     * Each pair of Music instances played at the same time will be represented by the concatenation
     *     '[' + firstMusic.toString() + ' ' + secondMusic.toString() + ']'
     * Each pair of Music instances played immediately after one another will be represented by the concatenation
     *     firstMusic.toString() + '+' + secondMusic.toString()
     *
     * The binary groupings and ordering of the above will preserve the structure from the Music's construction via
     * static factory methods.
     *
     * @return a string uniquely defining this music
     */