code: def test_to_lower(_nltk, text_data, tokenized_test_list_main):
      assert (
          to_list(to_lower(tokenized_test_list_main))
          == text_data["answer_key_lower"]
      ), "Output did not correctly convert documents to lowercase"
  
file_path: data-describe/tests/test_text_preprocessing.py
imports: import pytest
  import pandas
  from data_describe.text.text_preprocessing import tokenize, to_lower, remove_digits, remove_punct, remove_single_char_and_spaces, remove_stopwords, preprocess_texts, to_list, bag_of_words_to_docs, create_doc_term_matrix, create_tfidf_matrix, ngram_freq
label_desc: Use image node_id 3 for calling a global function with example usage: test_to_lower(_nltk, text_data, tokenized_test_list_main) without return types