title: How to concatenate two Sets of strings in Perl 6?
body: <p>Is there an idiomatic way or a built-in method to <a href="https://en.wikipedia.org/wiki/Concatenation#Concatenation_of_sets_of_strings" rel="noreferrer">concatenate</a> two <code>Sets</code> of strings? </p>
  
  <p>Here's what I want:</p>
  
  <pre class="lang-perl6 prettyprint-override"><code>&gt; my Set $left = &lt;start_ begin_&gt;.Set
  set(begin_ start_)
  &gt; my Set $right = &lt;end finish&gt;.Set
  set(end finish)
  &gt; my Set $left_right = ($left.keys X~ $right.keys).Set
  set(begin_end begin_finish start_end start_finish)
  </code></pre>
  
  <p>Or, if there are more than two of them:</p>
  
  <pre class="lang-perl6 prettyprint-override"><code>&gt; my Set $middle = &lt;center_ base_&gt;.Set
  &gt; my Set $all = ([X~] $left.keys, $middle.keys, $right.keys).Set
  set(begin_base_end begin_base_finish begin_center_end begin_center_finish start_base_end start_base_finish start_center_end start_center_finish)
  </code></pre>