code: def block_mask(block_lists):
      """
      Yields a flat iterable of booleans to mark the zeroth elements of the
      nested input ``block_lists`` in a flattened output.
  
      >>> list(block_mask([[1, 2], [3, 4], [5]]))
      [True, False, True, False, True]
      """
      for block in block_lists:
          if not block:
              continue
          yield True
          yield from (False for _ in block[1:])
  
imports: from __future__ import annotations
  import os
  from collections.abc import Mapping
  from io import BytesIO
  from warnings import catch_warnings, simplefilter, warn
  import numpy
  import pandas
  from fsspec.compression import compr
  from fsspec.core import get_fs_token_paths
  from fsspec.core import open
  from fsspec.core import open_files
  from fsspec.utils import infer_compression
  from pandas.api.types import CategoricalDtype, is_datetime64_any_dtype, is_float_dtype, is_integer_dtype, is_object_dtype
  from dask.base import tokenize
  from dask.bytes import read_bytes
  from dask.core import flatten
  from dask.dataframe.backends import dataframe_creation_dispatch
  from dask.dataframe.io.io import from_map
  from dask.dataframe.io.utils import DataFrameIOFunction
  from dask.dataframe.utils import clear_known_categories
  from dask.delayed import delayed
  from dask.utils import asciitable, parse_bytes
  from dask.dataframe.core import _Frame
label_desc: Use image node_id 4 for calling a global function with example usage: block_mask(block_lists) without return types