Counting in Tuple Relational Calculus

Given the following schema: - Staff (**sid**, sname) - Project (**pid**, pdesc, pfrom, pto) - Asset (**aid**, acat, adesc) - Workfor (**sid, pid**, wfrom, wto) - Assignment (**aid, sid, pid**, afrom, ato) Note: The **bold** attributes are the keys. Find the names of the staffs who worked or are working for more than one project, in Tuple Relational Calculus. I was thinking of using count() but it is not allowed in TRC (or according to my Prof). Any answers appreciated.