code: def convert_letters_to_indices(samples):
      """
      Convert Reber Grammar words to the list of indices where
      each index referes to specific letter.
  
      Parameters
      ----------
      samples : list of str
          List of words.
  
      Examples
      --------
      >>> convert_letters_to_indices(['XXXXVTTSSV', 'VXXVS'])
      array([array([3, 3, 3, 3, 1, 0, 0, 4, 4, 1]),
             array([1, 3, 3, 1, 4])], dtype=object)
      """
      index_samples = []
      for sample in samples:
          word = [avaliable_letters.index(letter) for letter in sample]
          index_samples.append(np.array(word))
      return np.array(index_samples)
  
imports: import math
  from random import choice, randint
  import numpy
label_desc: Use image node_id 3 for calling a global function with example usage: convert_letters_to_indices(samples) and returns: np