In YAML notation, if the contents of the variable x (which is of type Map<String,List<String>>)
are:
x:
2010: [Item 1,Item 2]
2011: [Item 9]
2012: [Item 6]
The following will do what you ask for:
<ul>
$x.keys:{
k | <li>$k$</li>
<ul>
<li>$x.(k);separator="</li>
<li>"$</li>
</ul>
}$</ul>
Note the $x.(k)$
notation.