title: How to remove multiple elements from Observable array without subscribing to it
body: <p>In my angular 4 app, I came across a scenario on removing few elements from an observable array containing a custom type. Here I have another Observable array of elements to be removed. Is there a way to remove elements of one Observable array from another without subscribing to them.</p>
  
  <p>For example, say if I have observable arrays of <strong>allCustomers</strong> and <strong>orderedCustomers</strong>.</p>
  
  <p>How to I get customers who aren't ordered anything. (allCustomers - orderedCustomers = ?)</p>
  
  <p>Thanks.</p>