Why forbidden to use a remote function inside a guard : <p>Why I can't to use <code>String</code> or other module in guard?</p>

<p>Code:</p>

<pre><code>def foo(s1, s2) when String.length(s1) == String.length(s2) do 
   # something
end
</code></pre>

<p>And how I can elegantly reformat such case, when I wish to use module functions?</p>
