@TechReport{NegAlgebratech,
  author =	 {F. Esponda and E. Trias and E.S. Ackley and S. Forrest},
  title =	 {A Relational Algebra for Negative Databases},
  institution =	 {University of New Mexico},
  year =	 {2007},
  number =	 {TR-CS-2007-18}
}