Add a few videos.

The videos are work in progress and may be replaced by newer versions
at any time. Hence, if possible, please use the stable links to refer
to these videos, since they will always point to the latest version.

For example, to link to the Sudoku video, please use:

    https://www.metalevel.at/prolog/videos/sudoku

This will automatically redirect to the latest version. See also #3.

prolog/clpfd.html

@@ -163,6 +163,15 @@
     Prolog programs also reason about integers.
 
     <br><br>
+
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="videos/integers"><img src="videos/t_integers.png" alt="Integers"></a>
+        </td>
+      </tr>
+    </table>
+
     <center><h2 id="constraint">Constraints</h2></center>
 
     Generally, an <i>n</i>-ary <b>constraint</b> is a <i>relation</i>

prolog/concepts.html

@@ -32,6 +32,15 @@
     and <tt>X=Y</tt> instead of <tt>+(a,b)</tt> and <tt>=(X,Y)</tt>,
     respectively.
 
+    <br><br>
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="videos/operators"><img src="videos/t_operators.png" alt="Operators"></a>
+        </td>
+      </tr>
+    </table>
+
     <center><h2 id="program">Programs</h2></center>
 
     A Prolog <b>program</b> is a set of <i>predicates</i>.
@@ -63,6 +72,16 @@
 
     <br><br>
 
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="videos/predicates"><img src="videos/t_predicates.png" alt="Predicates"></a>
+        </td>
+      </tr>
+    </table>
+
+    <br><br>
+
     A predicate is defined by a collection of <i>clauses</i>.
 
     <br><br>
@@ -74,6 +93,15 @@
       the whole predicate is&nbsp;true.
     </div>
 
+    <br><br>
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="videos/clauses"><img src="videos/t_clauses.png" alt="Clauses, Rules and Facts"></a>
+        </td>
+      </tr>
+    </table>
+
     <center><h3 id="rule">Rules</h3></center>
 
     A Prolog <b>rule</b> has the form:
@@ -338,6 +366,15 @@
     We can implement <tt>hailstone/2</tt> more efficiently
     with&nbsp;<a href="metapredicates#if_3"><tt>if_/3</tt></a>.
 
+    <br><br>
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="videos/collatz"><img src="videos/t_collatz.png" alt="Collatz Conjecture"></a>
+        </td>
+      </tr>
+    </table>
+
     <br><br><br>
     <b><a href="/prolog">More about Prolog</a></b>
 

prolog/data.html

@@ -84,6 +84,17 @@
     to&nbsp;<a href="optimization#tree">trees</a>. There is a
     <a href="sorting#order">standard&nbsp;order</a> on&nbsp;terms.
 
+    <br><br>
+
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="videos/terms"><img src="videos/t_terms.png" alt="Terms"></a>
+        </td>
+      </tr>
+    </table>
+
+
     <center><h2 id="list">Lists</h2></center>
 
     Prolog <b>lists</b> are a special case of <a href="#term"><i>terms</i></a>.
@@ -144,6 +155,16 @@
     <a href="dcg">DCG notation</a> is frequently used for
     describing&nbsp;lists.
 
+    <br><br>
+
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="videos/lists"><img src="videos/t_lists.png" alt="Lists"></a>
+        </td>
+      </tr>
+    </table>
+
     <center><h2 id="pair">Pairs</h2></center>
 
     By convention, the functor <tt>(-)/2</tt> is often used to denote

prolog/facets.html

@@ -192,6 +192,15 @@ <h2 id="logic"><center>Prolog is a logic programming language</center></h2>
 
     A <a href="purity">pure</a> Prolog program consists of a set
     of <a href="https://en.wikipedia.org/wiki/Horn_clause">Horn&nbsp;clauses</a>.
+
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="videos/horn_clauses"><img src="videos/t_horn_clauses.png" alt="Horn Clauses"></a>
+        </td>
+      </tr>
+    </table>
+
     Its execution can be regarded as a special case
     of <i>resolution</i>, which is an algorithm that is rooted in
     formal&nbsp;logic. Logically, when Prolog answers a query, it

prolog/metapredicates.html

@@ -19,6 +19,16 @@
     Prolog predicates that take <i>predicates</i> as arguments are
     called <b>higher-order predicates</b>, or <b>meta-predicates</b>.
 
+    <br><br>
+
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="videos/metapredicates"><img src="videos/t_metapredicates.png" alt="Meta-Predicates"></a>
+        </td>
+      </tr>
+    </table>
+
     <center><h2 id="call"><tt>call/N</tt></h2></center>
 
     The <tt>call/N</tt> family

prolog/optimization.html

@@ -145,6 +145,16 @@
 
     <br><br>
 
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="videos/map_colouring"><img src="videos/t_map_colouring.png" alt="Map Colouring"></a>
+        </td>
+      </tr>
+    </table>
+
+    <br><br>
+
     We can easily map this task to a combinatorial task
     over <i>integers</i>, by using one logical variable for each
     region, and one integer for each&nbsp;colour.

prolog/puzzles.html

@@ -37,6 +37,15 @@
     Raymond&nbsp;Smullyan's <i>What Is the Name of This&nbsp;Book?</i>
     and Maurice&nbsp;Kraitchik's <i>Mathematical&nbsp;Recreations</i>.
 
+    <br><br>
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="videos/knights_and_knaves"><img src="videos/t_knights_and_knaves.png" alt="Knights and Knaves"></a>
+        </td>
+      </tr>
+    </table>
+
     <br><br>
 
     <div class="example">

queens/queens.html

@@ -28,6 +28,16 @@ <h1>Solving N-queens with Prolog</h1>
 
     The task is to place N queens on an N&times;N chessboard in such a
     way that none of the queens is under attack.
+
+    <br><br>
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="/prolog/videos/n_queens"><img src="/prolog/videos/t_n_queens.png" alt="N-Queens"></a>
+        </td>
+      </tr>
+    </table>
+
     <br><br>
 
     <h2><center>Prolog Solution</center></h2>

sudoku/sudoku.html

@@ -17,6 +17,15 @@ <h2><center>Prolog solution</center></h2>
     solving <a href="/prolog/optimization">combinatorial</a>
     tasks like&nbsp;<i>Sudoku</i>.
 
+    <br><br>
+    <table>
+      <tr>
+        <td><i>Video</i>:</td>
+        <td><a href="/prolog/videos/sudoku"><img src="/prolog/videos/t_sudoku.png" alt="Sudoku"></a>
+        </td>
+      </tr>
+    </table>
+
     <br><br>
 
     For example, using <a href="/prolog/clpfd">constraints</a>, a