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 ddims(self):
      dimensions = []
      dimension_names = []
      for el in self:
          for dim in el.dimensions():
              if dim.name not in dimension_names:
                  dimensions.append(dim)
                  dimension_names.append(dim.name)
      return dimensions
  
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 10 for calling the Overlay obj's underlying member method code with example usage: obj.ddims() and returns: dimensions