title: Testing for the definedness of a variable in Scheme
body: <p>Besides regular definitions like <code>(define variable value)</code> Scheme also allows for uninitialized definitions like <code>(define variable)</code>, when the variable is bound to some value later on with the aid of <code>set!</code>.</p>
  
  <p>Are there procedures allowing to test whether a symbol is a defined (not necessarily initialized) variable? Something like <code>(defined? 'variable)</code> which should return <code>#t</code> if <code>variable</code> is defined and <code>#f</code> otherwise?</p>