ordered list 1, 1a...

Hi I have to produce an ordered list but some entries will be 1, 1a etc
How can I use <ol> numeric list which autonumber the list items and specify the alpha sublists?

e.g.
1 Text Text
1a Text Text
2 Text Text
3 Text Text
3a Text Text
3b Text Text
4

You can't have 1a unless you specify it yourself. The easiest way is:
<style type="text/css">
ol {
list-style: decimal;
}
ol ol {
list-style: lower-alpha;
}
</style>
...
<ol>
<li>Lorem ipsum.
<ol>
<li>Dolor sit amet.</li>
</ol>
</li>
<li>Lorem ipsum.
<ol>
<li>Dolor sit amet.</li>
</ol>
</li>
</ol>

thanks you