@Article{	  dl04tocs,
  author	= "Volker Diekert and Markus Lohrey",
  journal	= "Theory of Computing Systems",
  pages		= "133--156",
  title		= "Existential and Positive Theories of Equations in Graph
		  Products",
  volume	= "37",
  year		= "2004",
  doi		= "10.1007/s00224-003-1110-x"
}