Each map includes a collection of zero or more Layers rooted in its
reference frame (the layers list) as well as a collection of zero or more
child LayerMaps, which have reference frames that are defined relative to
this layer's reference frame (the childMaps dictionary).
A collection of layers in a hierarchical tree.
Each map includes a collection of zero or more Layers rooted in its reference frame (the layers list) as well as a collection of zero or more child LayerMaps, which have reference frames that are defined relative to this layer's reference frame (the childMaps dictionary).