class_docstr: A ChainMap groups multiple dicts (or other mappings) together
  to create a single, updatable view.
  
  The underlying mappings are stored in a list.  That list is public and can
  be accessed / updated using the *maps* attribute.  There is no other state.
  
  Lookups search the underlying mappings successively until a key is found.
  In contrast, writes, updates, and deletions only operate on the first
  mapping.
code: def __len__(self):
      # reuses stored hash values if possible
      return len(set().union(*self.maps))
  
imports: import sys
label_desc: Use image node_id 5 for calling the ChainMap obj's underlying member method code with example usage: obj.__len__() and returns: len