name = 'dup/2'
slug = 'duplicate the elements of a list'

description = '''\
<p><code>dup(L1, L2)</code>: the list <code>L2</code> is obtained from <code>L1</code> by duplicating every element.</p>
<pre>
?- dup([1,2], X).
  X = [1,1,2,2].
?- dup([1,2,3], X).
  X = [1,1,2,2,3,3].
</pre>'''

hint = {}