Modify blank URI

I am using this to modify get request in F5 to real server.

if { [HTTP::uri] starts_with "/supervisor" } {

HTTP::uri [string map {"/supervisor" "/admin"} [HTTP::uri]]

}

and it works OK.

but i need to change the get request to /user but only if URI is blank.

so abc.com/ will send get abc.com/user

I have tried this...

if { [HTTP::uri] starts_with "/" } {

HTTP::uri [string map {"/" "/user"} [HTTP::uri]]

}

this kind of works as i see get abc.com/user but it then adds /user to all URI's, so abc.com/admin becomes abc.com/useradmin

so i only need to add /user if the URI is blank,

any help appreciated as going bonkers...