  Talk given at "Quantum Integrable Systems and Geometry" (3-7 September 2012,
Olhao, Portugal) and based on joint works with S. Belliard, N. Crampe and D.
Simon, see arXiv:1009.4119, arXiv:1105.4119, arXiv:1106.3264 and
arXiv:1209.4269.
  This short note presents only the basic ideas of the technique, and does not
attend to give a general overview of the subject. Interested readers should
refer to the original papers and references therein.
