title: Expressing the double summation sequence in Raku
body: <p>How to express the <em>double variable double summation</em> sequence in Perl 6?</p>
  
  <p>For an example of <em>double variable double summation</em> sequence see this</p>
  
  <p><img src="https://i.ibb.co/Qmz7g4b/double-summation.png" alt="image"></p>
  
  <p>It must be expressed as is, i.e. without mathematically reducing the double summation into a single summation. Thank you.</p>