title: Add a list of multisets in python
body: <p>What is the pythonic way to merge a list of <code>multiset</code>s into a single multiset?</p>
  <p>For example,</p>
  <pre><code>from multiset import Multiset
  
  set1 = Multiset('aab')
  set2 = Multiset('abc')
  ls = [set1, set2]
  </code></pre>
  <p>Based on using <code>set</code>s my guess was:</p>
  <pre><code>Multiset.add(*ls)
  </code></pre>
  <p>This gives an error:</p>
  <pre><code>Traceback (most recent call last):
  File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
  File &quot;/usr/local/lib/python3.7/site-packages/multiset.py&quot;, line 931, in add
  self._elements[element] += multiplicity
  TypeError: unhashable type: 'Multiset'
  </code></pre>
  <p>Is there a better way than a <code>for</code> loop, as below?</p>
  <pre><code>def add_multisets(ls):
      result = Multiset()
      for i in ls:
          result += i
      return result
  
  &gt;&gt;&gt; add_multisets(ls)
  Multiset({'a': 3, 'b': 2, 'c': 1})
  </code></pre>