code: public T visitTupleElement(TupleElement elm, C context) {
          visitElement(elm.getValue(), context);
          return null;
      }
docs: Visit a TupleElement. This method will be called for
  every node in the tree that is a TupleElement.
  
  @param elm     the ELM tree
  @param context the context passed to the visitor
  @return the visitor result