title: MATLAB/Octave: feval with vector input
body: <p>I want to put a function in feval(f,x) that has a vector as input
  e.g.</p>
  
  <pre><code>function [ ret ] = f (x)
      ret = x(1)^2 - x(2)^2;
  end
  </code></pre>
  
  <p>and</p>
  
  <pre><code>x = [1,2]
  </code></pre>
  
  <p>but octave always gives an error code:</p>
  
  <pre><code>`x' undefined near line 6 column 18
  evaluating argument list element number 1
  evaluating argument list element number 1
  </code></pre>
  
  <p>It seems feval can only evaluate numbers and not vectors. Is there any way to do this?</p>