title: Merge 2 lists according to values in a boolean list
body: <p>I have a method of achieving this which also explains my question.</p>
  <pre><code>a:1 2 3 4;
  b:5 6 7;
  cond:1101001b;
  comb:(count cond) # 0N;
  comb[where cond]:a;
  comb[where not cond]:b
  </code></pre>
  <p>But q has so many utilities for manipulating lists, I am wondering if there is a more direct way of doing this.</p>