title: How to plot a surface over a constrained domain?
body: <p>How would I plot the surface:      </p>
  
  <p><code>z=(1+x^2)/(1+y^2)</code> over the region <code>|x|+|y|&lt;=2</code> ?</p>
  
  <p>I cannot get the surface constrained to the rhombus/square-shaped region.</p>