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+unfinished:Group
+<p>
+Fill in the grid with the letters shown to the top and left of it, so
+that the full grid is a valid
+<a href="http://en.wikipedia.org/wiki/Cayley_table">Cayley table</a>
+for a
+<a href="http://en.wikipedia.org/wiki/Group_(mathematics)">group</a>.
+<p>
+If you don't already know what a group is, I don't really recommend
+trying to play this game. But if you want to try anyway, the above is
+equivalent to saying that the following conditions must be satisfied:
+<ul>
+<li>
+<strong>Latin square</strong>. Every row and column must contain
+exactly one of each letter.
+<li>
+<strong>Identity</strong>. There must be some letter <i>e</i> such
+that, for all <i>a</i>, the letter in row <i>e</i> column <i>a</i> and
+the one in row <i>a</i> column <i>e</i> are both <i>a</i>. In the
+default mode, this letter is always <i>e</i> and its row and column
+are filled in for you; by reconfiguring the game using the Type menu,
+you can select a mode in which you have to work out which letter is
+the identity.
+<li>
+<strong>Inverses</strong>. For every letter <i>a</i>, there must be
+some letter <i>b</i> (which may sometimes be the same letter
+as <i>a</i>) such that the letters in row <i>a</i> column <i>b</i> and
+in row <i>b</i> column <i>a</i> are both the identity letter (as
+defined above).
+<li>
+<strong>Associativity</strong>. For every combination of
+letters <i>a</i>, <i>b</i>, and <i>c</i>, denote the letter in
+row <i>a</i> column <i>b</i> by <i>d</i>, and the one in row <i>b</i>
+column <i>c</i> by <i>e</i>. Then the letters in row <i>d</i>
+column <i>c</i> and in row <i>a</i> column <i>e</i> must be the same.
+</ul>
+<p>
+To place a letter, click in a square to select it, then type the
+letter on the keyboard. To erase a letter, click to select a square
+and then press Backspace.
+<p>
+Right-click in a square and then type a letter to add or remove the
+number as a pencil mark, indicating letters that you think
+<em>might</em> go in that square.
+<p>
+You can rearrange the order of elements in the rows and columns by
+dragging the column or row headings back and forth. (The rows and
+columns will stay in sync with each other.) Also,
+left-clicking <em>between</em> two row or column headings will add or
+remove a thick line between those two rows and the corresponding pair
+of columns (which is useful if you're considering a subgroup and its
+cosets).