[Serusers] how can I look up the username in the location table?

I want to look up the username in the location table during the method==register if block. The reason I wish to do this is to see if there is another location currently registered in the database for this particular username. How can I do this? Do I have to use exec_msg() or exec_dset(), or does lookup() work? I've tried lookup("location") but it failed to work, it seems. Can anyone help me out? Here is the block of code that I'm trying to use: if (method=="REGISTER") { if (!search("^Expires: 0")) { if (lookup("location")) { sl_send_reply("200","Another registration."); } else { sl_send_reply("200", "First registration."); } } else { sl_send_reply("200", "Unregistration."); } save_noreply("location"); break; } Note that it is sending the proper replies for "First registrations" and "Unregistrations," but not for "Another registrations." Thanks in advance, Dana