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copied!<p>I need to do a little more research, but here's two ways; The catch is, you don't actually use the <code>Orderless</code> attribute. Simon should get half-credit for the technique of using pattern-matching with a condition from <a href="https://stackoverflow.com/questions/3947071/controlling-order-of-variables-in-an-expression/3948995#3948995">his answer to question 1</a> you referenced. </p> <p>Messing with <code>Order</code>, <code>OrderedQ</code> and <code>Sort</code> won't get you anywhere, because <code>Orderless</code> functions sort with <em>equivalence</em> to those methods, but does not actually sort <em>with</em> them.</p> <p>The first method, from Simon's answer, is to use a <a href="http://reference.wolfram.com/mathematica/ref/Condition.html" rel="nofollow noreferrer"><code>Condition</code></a>:</p> <pre><code>In[1]:= ClearAll[f, g] In[2]:= f[stuff__] /; ! OrderedQ[{stuff}, Greater] := f[Sequence@@Sort[{stuff}, Greater]] In[3]:= f[1, 2, 3] Out[3]= f[3, 2, 1] In[4]:= f[3, 2, 1] Out[4]= f[3, 2, 1] </code></pre> <p>That definition of f[stuff__] is evaluated if the order of the arguments is not the order desired, and then transformed into that order. </p> <p>Another technique is to get a little more low-level and use <a href="http://reference.wolfram.com/mathematica/ref/%24Pre.html" rel="nofollow noreferrer"><code>$Pre</code></a> or <a href="http://reference.wolfram.com/mathematica/ref/%24PreRead.html" rel="nofollow noreferrer"><code>$PreRead</code></a>:</p> <pre><code>In[5]:= $Pre = Function[{expr}, expr /. g[stuff__] :> g[Sequence @@ Sort[{stuff}, Greater]]]; In[6]:= g[1, 2, 3] Out[6]= g[3, 2, 1] In[7]:= g[3, 2, 1] Out[7]= g[3, 2, 1] </code></pre> <p>There are some issues with both of these approaches as they stand, for example, that particular <code>$Pre</code> function doesn't play well with e.g. <code>HoldForm[g[1,2,3]]</code>. </p> <p>So, that's something to play with for now. I hope to update after I do a little more research into this.</p>

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