title: Which Ruby classes support .clone?
body: <p>Ruby defines <code>#clone</code> in <strong>Object</strong>.
  To my suprise, some classes raise Exceptions when calling it.
  I found <strong>NilClass</strong>, <strong>TrueClass</strong>, <strong>FalseClass</strong>, <strong>Fixnum</strong> having this behaviour.</p>
  
  <p>1) Does a complete list of classes (at least core-classes) exist, which do not allow <code>#clone</code> ?
  Or is there a way to detect if a specific class supports <code>#clone</code> ?</p>
  
  <p>2) What is wrong with <code>42.clone</code> ?</p>