title: check if expression contains symbol
body: <p>I would like to find out programatically if a SymPy expression <em>contains</em> a symbol. E.g., for </p>
  
  <pre class="lang-cpp prettyprint-override"><code>import sympy
  x = sympy.Symbol('x')
  y = sympy.Symbol('y')
  a = 4 + x**2 + y
  b = 4 + y**2
  </code></pre>
  
  <p><code>a</code> contains both <code>x</code> and <code>y</code>, <code>b</code> contains only <code>y</code>.</p>