how to get the length of a 2-d counter
<p>build the counter:</p>

<pre><code>class Counters:
    def __init__(self,cats)
        self.form = collections.defaultdict(collections.Counter)
</code></pre>

<p>I get the length of the first dimension:</p>

<pre><code>len(counters.form.items())
</code></pre>

<p>How to get the length of the second dimension?</p>
