/** Add a name usage to the simplifier's internal cache */
 protected void addUsage(Symbol sym) {
   Name n = sym.getSimpleName();
   List<Symbol> conflicts = nameClashes.get(n);
   if (conflicts == null) {
     conflicts = List.nil();
   }
   if (!conflicts.contains(sym)) nameClashes.put(n, conflicts.append(sym));
 }