code: ///////////////////////////////////////////////////////////////////////////
  //                   __                _      _   ________               //
  //                  / /   ____  ____ _(_)____/ | / / ____/               //
  //                 / /   / __ \/ __ `/ / ___/  |/ / / __                 //
  //                / /___/ /_/ / /_/ / / /__/ /|  / /_/ /                 //
  //               /_____/\____/\__, /_/\___/_/ |_/\____/                  //
  //                           /____/                                      //
  //                                                                       //
  //               The Next Generation Logic Library                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  //                                                                       //
  //  Copyright 2015-20xx Christoph Zengler                                //
  //                                                                       //
  //  Licensed under the Apache License, Version 2.0 (the "License");      //
  //  you may not use this file except in compliance with the License.     //
  //  You may obtain a copy of the License at                              //
  //                                                                       //
  //  http://www.apache.org/licenses/LICENSE-2.0                           //
  //                                                                       //
  //  Unless required by applicable law or agreed to in writing, software  //
  //  distributed under the License is distributed on an "AS IS" BASIS,    //
  //  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or      //
  //  implied.  See the License for the specific language governing        //
  //  permissions and limitations under the License.                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  
  package org.logicng.transformations;
  
  import org.logicng.datastructures.ubtrees.UBTree;
  import org.logicng.formulas.Formula;
  import org.logicng.formulas.Literal;
  
  import java.util.ArrayList;
  import java.util.List;
  import java.util.Map;
  import java.util.SortedMap;
  import java.util.SortedSet;
  import java.util.TreeMap;
  
  /**
   * A superclass for subsumptions (CNF or DNF).
   * @version 2.0.0
   * @since 1.5.0
   */
  public abstract class Subsumption {
  
      /**
       * Generates a UBTree from the formulas operands (clauses in CNF, minterms in DNF)
       * where all subsumed operands are already deleted.
       * @param formula the formula (must be an n-ary operator and CNF or DNF)
       * @return the UBTree with the operands and deleted subsumed operands
       */
      protected static UBTree<Literal> generateSubsumedUBTree(final Formula formula) {
          final SortedMap<Integer, List<SortedSet<Literal>>> mapping = new TreeMap<>();
          for (final Formula term : formula) {
              mapping.computeIfAbsent(term.literals().size(), k -> new ArrayList<>()).add(term.literals());
          }
          final UBTree<Literal> ubTree = new UBTree<>();
          for (final Map.Entry<Integer, List<SortedSet<Literal>>> entry : mapping.entrySet()) {
              for (final SortedSet<Literal> set : entry.getValue()) {
                  if (ubTree.firstSubset(set) == null) {
                      ubTree.addSet(set);
                  }
              }
          }
          return ubTree;
      }
  }
  