code: def expr_labelfunc(leaf_renderer=str, fallback=str):
      
  
      def _labelfunc(expr, is_leaf):
          if is_leaf:
              label = leaf_renderer(expr)
          elif isinstance(expr, Expression):
              if len(expr.kwargs) == 0:
                  label = expr.__class__.__name__
              else:
                  label = "%s(..., %s)" % (
                      expr.__class__.__name__,
                      ", ".join([
                          "%s=%s" % (key, val)
                          for (key, val) in expr.kwargs.items()]))
          else:
              label = fallback(expr)
          return label
  
      return _labelfunc
docs: Factory for function ``labelfunc(expr, is_leaf)``
  
      It has the following behavior:
  
      * If ``is_leaf`` is True, return ``leaf_renderer(expr)``.
  
      * Otherwise,
  
        - if `expr` is an Expression, return a custom string similar to
          :func:`~qnet.printing.srepr`, but with an ellipsis for ``args``
        - otherwise, return ``fallback(expr)``