code: function add (a, b) {
    var fifths = a[0] + b[0]
    var octaves = a[1] === null || b[1] === null ? null : a[1] + b[1]
    return [fifths, octaves]
  }
docs: Add two pitches. Can be used to tranpose pitches.
  
  @param {Array} first - first pitch
  @param {Array} second - second pitch
  @return {Array} both pitches added
  
  @example
  operator.add([3, 0, 0], [4, 0, 0]) // => [0, 0, 1]