class_docstr: An Overlay consists of multiple Elements (potentially of
  heterogeneous type) presented one on top each other with a
  particular z-ordering.
  
  Overlays along with elements constitute the only valid leaf types of
  a Layout and in fact extend the Layout structure. Overlays are
  constructed using the * operator (building an identical structure
  to the + operator).
code: def group(self, group):
      if not sanitize_identifier.allowable(group):
          raise ValueError(
              f"Supplied group {group} contains invalid characters."
          )
      else:
          self._group = group
  
imports: from functools import reduce
  import numpy
  import param
  from .dimension import Dimension, Dimensioned, ViewableElement, ViewableTree
  from .layout import AdjointLayout, Composable, Layout, Layoutable
  from .ndmapping import UniformNdMapping
  from .util import dimensioned_streams, sanitize_identifier, unique_array
label_desc: Use image node_id 7 for calling the Overlay obj's underlying member method code with example usage: obj.group(group) without return types