code: def infer_chromosomes(blockdf: DataFrame) -> List[str]:
      """
      Extracts chromosomes from a once- or twice-reduced block DataFrame.
  
      Args:
          blockdf : Spark DataFrame representing a once- or twice-reduced block matrix.
  
      Returns:
          List of chromosomes.
      """
      # Regex captures the chromosome name in the header
      # level 1 header: chr_3_block_8_alpha_0_label_sim100
      # level 2 header: chr_3_alpha_0_label_sim100
      chromosomes = [
          r.chromosome
          for r in blockdf.select(
              f.regexp_extract(
                  "header", r"^chr_(.+?)_(alpha|block)", 1
              ).alias("chromosome")
          )
          .distinct()
          .collect()
      ]
      print(f"Inferred chromosomes: {chromosomes}")
      return chromosomes
  
imports: import itertools
  import math
  import re
  import warnings
  from typing import Any, Dict, Iterable, List, Tuple
  import numpy
  import pandas
  import pyspark.sql.functions
  import scipy.optimize
  from nptyping import Float, Int, NDArray
  from pyspark.sql import DataFrame, Row
  from pyspark.sql.window import Window
  from typeguard import typechecked
label_desc: Use image node_id 29 for calling a global function with example usage: infer_chromosomes(blockdf) and returns: chromosomes