ability> pairs does not, strictly speaking, have to be a function and it can
 contain null values.  The pmf created by the constructor will combine probabilities of equal values and
 will treat null values as equal.  For example, if the list of pairs &lt;"dog", 0.2&gt;, &lt;null, 0.1&gt;,
 &lt;"pig", 0.2&gt;, &lt;"dog", 0.1&gt;, &lt;null, 0.4&gt; is provided to the constructor, the resulting
 pmf will assign mass of 0.5 to null, 0.3 to "dog" and 0.2 to null.</p></div>
<dl><dt><span class="strong">Since:</sp