  However, these intermediate problems don’t tend to show up.   This phenomenon has received a great deal of attention recently via the dichotomy conjecture of Feder and Vardi [Feder and Vardi, 1999]. Schaefer introduced this work with an insightful paper in which he studied all variants of the boolean satisfiability problem [Schaefer, 1978].   He proved that each such problem     was either NP  complete or in P, thus Ladner’s intermediate problems cannot occur among such satisfiability problems.  Since there were just these two possibilities, Schaefer called this a dichotomy.  