input: Write a function to extract all the words starting with a specific letter from a given string. The function should take two parameters: the string to be processed and the letter to search for. If no words start with the specified letter, the function should return an empty list.
  
  def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list
  
  assert extract_words('This is a sample sentence to test the program', 's')==['sample', 'sentence', 'test', 'the']
  assert extract_words('Many people enjoy walking in the park', 'e')==['enjoy', 'enjoy', 'the']
  assert extract_words('The quick brown fox jumps over the lazy dog', 'a')==['over', 'the']
  assert extract_words('Apples are sweet and so are oranges', 'z')==[]
  assert extract_words('', 'a')==[]
output: def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w.startswith(letter):
              words_list.append(w)
      return words_list