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.Graph;
  import edu.cmu.tetrad.graph.Node;
  
  import java.util.List;
  
  /**
   * Specifies what coefs as a legal pair of edges X---Y---Z for purposes of calculating possible d-separation sets for
   * the FCI algorithm. In this case, legal initial edges are those adjacent to initial nodes, and legal pairs of edges
   * are those for which either X-->Y<--Z or X is adjacent to Z--i.e. X, Y, and Z form a triangle. (It is assumed (and
   * checked) that is adjacent to Y and Y is adjacent to Z.)
   *
   * @author Tyler Gibson
   */
  class IonLegalPairs implements LegalPairs {
  
      /**
       * Graph with respect to which graph properties are tested.
       */
      private Graph graph;
  
      /**
       * Constructs a new legal pairs object. See class level doc.
       *
       * @param graph The graph with respect to which legal pairs will be tested.
       */
      public IonLegalPairs(Graph graph) {
          if (graph == null) {
              throw new NullPointerException();
          }
  
          this.graph = graph;
      }
  
      /**
       * @return true iff x is adjacent to y.
       */
      public boolean isLegalFirstEdge(Node x, Node y) {
          return this.graph.isAdjacentTo(x, y);
      }
  
      /**
       * @throws IllegalArgumentException if x is not adjacent to y or y is not adjacent to z.
       */
      public boolean isLegalPair(Node x, Node y, Node z, List<Node> c, List<Node> d) {
          if (!(graph.isAdjacentTo(x, y)) || !(graph.isAdjacentTo(y, z))) {
              throw new IllegalArgumentException();
          }
          //noinspection SimplifiableIfStatement                
          if (graph.isDefCollider(x, y, x) || graph.isAdjacentTo(x, z)) {
              return true;
          }
  
          return this.graph.isUnderlineTriple(x, y, z);
      }
  }
  
  
  
  
  
  