content: /* 
   * Kodkod -- Copyright (c) 2005-present, Emina Torlak
   *
   * Permission is hereby granted, free of charge, to any person obtaining a copy
   * of this software and associated documentation files (the "Software"), to deal
   * in the Software without restriction, including without limitation the rights
   * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   * copies of the Software, and to permit persons to whom the Software is
   * furnished to do so, subject to the following conditions:
   *
   * The above copyright notice and this permission notice shall be included in
   * all copies or substantial portions of the Software.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
   * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
   * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
   * THE SOFTWARE.
   */
  package kodkod.engine.satlab;
  
  /**
   * Provides an interface to a SAT solver that can generate
   * proofs of unsatisfiability.
   * 
   * @specfield variables: set [1..)
   * @specfield clauses: set Clause
   * @specfield resolvents: set Clause  
   * @invariant all i: [2..) | i in variables => i-1 in variables
   * @invariant all c: clauses + resolvents | all lit: c.lits | lit in variables || -lit in variables
   * @invariant all c: clauses + resolvents | all disj i,j: c.lits | abs(i) != abs(j)
   * @author Emina Torlak
   */
  public interface SATProver extends SATSolver {
  	
  	/**
  	 * Returns a resolution-based proof of  unsatisfiability of this.clauses.
  	 * @requires {@link SATSolver#solve()} has been called, and it returned false
  	 * @return { t: ResolutionTrace | t.prover = this }
  	 * @throws IllegalStateException  {@link SATSolver#solve()} has not been called, 
  	 * or the last call to {@link SATSolver#solve()} returned true
  	 */
  	public ResolutionTrace proof();
  
  	/**
  	 * Uses the given reduction strategy to remove irrelevant clauses from 
  	 * the set of unsatisfiable clauses stored in this prover.  
  	 * A clause c is irrelevant iff this.clauses - c is unsatisfiable.
  	 * The removal algorithm works as follows:
  	 * <pre>
  	 * for (IntSet next = strategy.next(this.proof()); !next.isEmpty(); next = strategy.next(this.proof())) {
  	 *  let oldClauses = this.clauses, oldResolvents = this.resolvents
  	 *  clear this.clauses
  	 *  clear this.resolvents
  	 *  for(Clause c : this.proof().elts[next]) {
  	 *    if (no c.antecedents)
  	 *      add c to this.clauses
  	 *    else
  	 *      add c to this.resolvents
  	 *  }
  	 *  if (this.solve()) {
  	 *   this.clauses = oldClauses 
  	 *   this.resolvents = oldResolvents
  	 *  }
  	 * }
  	 * </pre>
  	 * @requires {@link SATSolver#solve()} has been called, and it returned false
  	 * @ensures modifies this.clauses and this.resolvents according to the algorithm described above 
  	 * @throws IllegalStateException  {@link SATSolver#solve()} has not been called, 
  	 * or the last call to {@link SATSolver#solve()} returned true
  	 * @see ReductionStrategy
  	 */
  	public void reduce(ReductionStrategy strategy);
  	
  }
  