I know this should be super easy but I can't get my head about how to do this in the sapphire framework.

I have a control block:

<a href="<% control Level(1) %>$Prefix<% end_control %>SomePage">SomePage</a>

As you can see it simply goes to the top level page and gets the $Prefix field.
My question is how can I make a function that will do the same thing only with one template command. Something like:

<a href="{$WritePrefix}SomePage">SomePage</a>

Or better yet, is there a way to get the prefix from the top level and place it in every <a href=""> ?