code: def update_backends(cls, id_mapping, custom_trees, backend=None):
          
          backend = Store.current_backend if backend is None else backend
          # Update the custom option entries for the current backend
          Store.custom_options(backend=backend).update(custom_trees)
  
          # Propagate option ids for non-selected backends
          for b in Store.loaded_backends():
              if b == backend:
                  continue
              backend_trees = Store._custom_options[b]
              for (old_id, new_id) in id_mapping:
                  tree = backend_trees.get(old_id, None)
                  if tree is not None:
                      backend_trees[new_id] = tree
docs: Given the id_mapping from previous ids to new ids and the new
          custom tree dictionary, update the current backend with the
          supplied trees and update the keys in the remaining backends to
          stay linked with the current object.