title: How to concatenate an optional array to another array in Swift?
body: <p>I know this sounds ridiculous but I am seriously confused now.
  Basically I have two vars, one is <code>[Data]</code> and one is <code>Array&lt;Data&gt;?</code> </p>
  
  <p>I need to combine these two.
  I've tried do <code>var1+var2</code>, which is giving me errors say can't do binary operations on these two. 
  So I googled, I can use append method, now there comes more questions:</p>
  
  <ol>
  <li>the append method is crossed out in autofill, so I am not sure if I should use it.</li>
  <li>Even if I try to use it (while it's being crossed out), it is still giving me errors </li>
  </ol>
  
  <p>Here is the code and errors I get:</p>
  
  <pre><code>var a1:[Data] //a return value from other function
  var a2:Array&lt;Data&gt;? //a parameter that's passed in
  
  a1.append(contentsOf:a2) //Cannot use mutating member on immutable value: function call returns immutable value
  a1+a2 //Binary operator '+' cannot be applied to operands of type 'Array&lt;Data&gt;' and 'Array&lt;Data&gt;?'
  </code></pre>
  
  <p>Both arrays can be empty, how can I concatenate these two arrays?</p>