code: public T visitInstance(Instance elm, C context) {
          for (InstanceElement element : elm.getElement()) {
              visitInstanceElement(element, context);
          }
          return null;
      }
docs: Visit a Instance. This method will be called for
  every node in the tree that is a Instance.
  
  @param elm     the ELM tree
  @param context the context passed to the visitor
  @return the visitor result