Combine the outer and inner keys of nested dictionaries into a single
    ordering.

def flattened_nested_key_indices(nested_dict):
    """
    Combine the outer and inner keys of nested dictionaries into a single
    ordering.
    """
    outer_keys, inner_keys = collect_nested_keys(nested_dict)
    combined_keys = list(sorted(set(outer_keys + inner_keys)))
    return {k: i for (i, k) in enumerate(combined_keys)}