code: function notBetween(first, second) {
    return source => xs.merge(
      source.endWhen(first),
      first.map(() => source.compose(dropUntil(second))).flatten()
    )
  }
docs: source: --a--b----c----d---e-f--g----h---i--j-----
  first:  -------F------------------F---------------
  second: -----------------S-----------------S------
  notBetween
  output: --a--b-------------e-f--g-----------j-----