Procedure DPLL ( )
(1)preprocess ( );
(2)if     then  return SAT;
(3)if     then  return UNSAT;
(4)  choosevariable ( );
(5)if ( is existential)
(6)then  return DPLL ( ) or DPLL ( )
(7)if (v is universal)
(8)then  return DPLL ( ) and DPLL ( )
Algorithm 1: DPLL for solving QBF.