ject</span>

<span class="sd">    Raises:</span>
<span class="sd">      ValueError if associative has no associative attribute</span>
<span class="sd">    &quot;&quot;&quot;</span>
  
<span class="c"># choosing the name ID creates an identity object. ID*b = b for all Coord elements b.</span>
<span class="c"># could implement the identity Field in __call__</span>

<span class="c"># Metric could be a class. Objects of this class could be constructed by a method of the Coord class (Coord objects then spawn met