code: ///////////////////////////////////////////////////////////////////////////////
  // For information as to what this class does, see the Javadoc, below.       //
  // Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006,       //
  // 2007, 2008, 2009, 2010, 2014, 2015 by Peter Spirtes, Richard Scheines, Joseph   //
  // Ramsey, and Clark Glymour.                                                //
  //                                                                           //
  // This program is free software; you can redistribute it and/or modify      //
  // it under the terms of the GNU General Public License as published by      //
  // the Free Software Foundation; either version 2 of the License, or         //
  // (at your option) any later version.                                       //
  //                                                                           //
  // This program is distributed in the hope that it will be useful,           //
  // but WITHOUT ANY WARRANTY; without even the implied warranty of            //
  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the             //
  // GNU General Public License for more details.                              //
  //                                                                           //
  // You should have received a copy of the GNU General Public License         //
  // along with this program; if not, write to the Free Software               //
  // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA //
  ///////////////////////////////////////////////////////////////////////////////
  
  package edu.cmu.tetrad.search;
  
  import edu.cmu.tetrad.graph.GraphNode;
  import edu.cmu.tetrad.graph.Node;
  import edu.cmu.tetrad.util.TetradSerializable;
  
  import java.util.ArrayList;
  import java.util.List;
  
  /**
   * Represents a sextad of variables.
   */
  public class Sextad implements TetradSerializable {
      static final long serialVersionUID = 23L;
  
      private Node i;
      private Node j;
      private Node k;
      private Node l;
      private Node m;
      private Node n;
  
      public Sextad(Node i, Node j, Node k, Node l, Node m, Node n) {
          testDistinctness(i, j, k, l, m, n);
          this.i = i;
          this.j = j;
          this.k = k;
          this.l = l;
          this.m = m;
          this.n = n;
      }
  
      public Sextad(Node[] nodes) {
          if (nodes.length != 6) throw new IllegalArgumentException("Must provide exactly 6 nodes.");
  
          this.i = nodes[0];
          this.j = nodes[1];
          this.k = nodes[2];
          this.l = nodes[3];
          this.m = nodes[4];
          this.n = nodes[5];
  
          testDistinctness(i, j, k, l, m, n);
  
      }
  
      /**
       * Generates a simple exemplar of this class to test serialization.
       */
      public static Sextad serializableInstance() {
          Node i = new GraphNode("i");
          Node j = new GraphNode("j");
          Node k = new GraphNode("k");
          Node l = new GraphNode("l");
          Node m = new GraphNode("m");
          Node n = new GraphNode("n");
          return new Sextad(i, j, k, l, m, n);
      }
  
      private void testDistinctness(Node i, Node j, Node k, Node l, Node m, Node n) {
          if (i == j || i == k || i == l || i == m || i == n) {
              throw new IllegalArgumentException("Nodes not distinct.");
          }
  
          if (j == k || j == l || j == m || j == n) {
              throw new IllegalArgumentException("Nodes not distinct.");
          }
  
          if (k == l || k == m || k == n) {
              throw new IllegalArgumentException("Nodes not distinct.");
          }
  
          if (l == m || l == n) {
              throw new IllegalArgumentException("Nodes not distinct.");
          }
  
          if (m == n) {
              throw new IllegalArgumentException("Nodes not distinct.");
          }
      }
  
      public Node getI() {
          return i;
      }
  
      public Node getJ() {
          return j;
      }
  
      public Node getK() {
          return k;
      }
  
      public Node getL() {
          return l;
      }
  
      public Node getM() {
          return m;
      }
  
      public Node getN() {
          return n;
      }
  
      public int hashCode() {
          int hash = 17 * i.hashCode() * j.hashCode() * k.hashCode();
          hash += 29 * l.hashCode() * m.hashCode() * n.hashCode();
  
          return hash;
      }
  
      public boolean equals(Object o) {
          if (!(o instanceof Sextad)) throw new IllegalArgumentException();
          if (o == null) return false;
          Sextad sextad = (Sextad) o;
  
          boolean leftEquals = i == sextad.i && j == sextad.j && k == sextad.k ||
                  i == sextad.i && j == sextad.k && k == sextad.j ||
                  i == sextad.j && j == sextad.i && k == sextad.k ||
                  i == sextad.j && j == sextad.k && k == sextad.i ||
                  i == sextad.k && j == sextad.i && k == sextad.j ||
                  i == sextad.k && j == sextad.j && k == sextad.i;
  
          boolean rightEquals = l == sextad.l && m == sextad.m && n == sextad.n ||
                  l == sextad.l && m == sextad.n && n == sextad.m ||
                  l == sextad.m && m == sextad.l && n == sextad.n ||
                  l == sextad.m && m == sextad.n && n == sextad.l ||
                  l == sextad.n && m == sextad.l && n == sextad.m ||
                  l == sextad.n && m == sextad.m && n == sextad.l;
  
          return leftEquals && rightEquals;
      }
  
      public String toString() {
          return "<" + i + ", " + j + ", " + k + "; " + l + ", " + m + ", " + n + ">";
      }
  
      public List<Node> getNodes() {
          List<Node> nodes = new ArrayList<>();
          nodes.add(i);
          nodes.add(j);
          nodes.add(k);
          nodes.add(l);
          nodes.add(m);
          nodes.add(n);
          return nodes;
      }
  }
  
  
  