@Article{	  dor94,
  author	= "Volker Diekert and Edward Ochma{\'n}ski and Klaus
		  Reinhardt",
  title		= "On confluent semi-commutation systems --- decidability and
		  complexity results",
  journal	= "Information and Computation",
  year		= "1994",
  volume	= "110",
  pages		= "164--182",
  doi		= "10.1006/inco.1994.1029"
}