title: Create a combination on input in Spock test
body: <p>In my Spock test this is my current where block </p>
  
  <pre><code>where:
          [A,B,C] &lt;&lt; 
          [
              ["A1","B1","C1"],
              ["A1","B1","C2"],
          ]
  </code></pre>
  
  <p>I am trying to simplify this since my input for A and B is always same. I am trying to use combinations() but have been unsuccessful.</p>
  
  <p>I want something like this which does same job as a code above.</p>
  
  <pre><code>where:
          [[A,B],[C]] &lt;&lt;                 // incorrect
          [
              [["A1","B1"],["C1","C2"]].combinations()  // incorrect
          ]   
  </code></pre>
  
  <p>Currently I get NPE </p>