op removestudent
   in: r: GradeRoster, name: String, ssnum: String;
   out: r': GradeRoster;
   pre: (* the GradeRoster must not empty. If the GradeRoster is empty then
           removes student will return the same list as pass in *)
	(* if (r=nil) return r; -- COMMENT: you probably want the following: *)
	  r != nil;
   post: (* Search through the GradeRoster until find a match with name and
            ssnum. If found the name will be deleted, otherwise search through
            until the GradeRoster exhausted and then return r'. *);
end removestudent;