<var>n</var>, <var>i</var>)</code> returns the <em>n</em>-degree
permutation which is at the <em>i</em>th position (from 1 to <em>n</em>!) in
the lexicographic ordering of the permutations.
</p>
<p><code>perms(<var>n</var>, <var>i</var>, <var>j</var>)</code> returns a list of the <em>n</em>-degree
permutations between positions <var>i</var> and <var>j</var> in the lexicographic
ordering of the permutations.
</p>
<p>The sequence of permutations in Trotter-Johnson order starts with the
identity permutation an