title: How to write to an Element in a Set?
body: <p>With arrays you can use a subscript to access <code>Array</code> <code>Element</code>s directly.  You can read or write to them. With Sets I am not sure of a way to write its Elements.</p>
  <p>For example, if I access a set element matching a condition I'm only able to read the element. It is passed by copy and I can't therefore write to the original.</p>
  <p>For example:</p>
  <pre><code>
  columns.first(
      where: { 
          $0.header.last == Character(String(i))
      }
  )?.cells.append(value: addValue)
  
  
  // ERROR: Cannot use mutating member on immutable value: function call returns immutable value
  </code></pre>