do a better job recognizing and bracketing operators

src/prolog/heap_print.rs

@@ -536,7 +536,7 @@ impl<'a, Outputter: HCValueOutputter> HCPrinter<'a, Outputter>
     fn print_atom(&mut self, atom: &ClauseName) {
         push_space_if_amb!(self, atom.as_str(), {
             match atom.as_str() {
-                "" => self.append_str("''"),
+                "''" => self.append_str("''"),
                 s => self.print_op_addendum(s)
             }
         });