docs: function transpose (interval, pitch) {
    if (arguments.length === 1) return function (p) { return transpose(interval, p) }
    var i = notation.arr(interval)
    var p = notation.arr(pitch)
    if (!i || !p) return null
    var r = p.length === 3 ? op.add(p, i) : op.add(i, p)
    return Array.isArray(pitch) ? r : notation.str(r)
  }