code: def StrSubstr(start_idx, count, initial_string):
      
      new_value = initial_string.value[start_idx.value:start_idx.value + count.value]
      return StringV(new_value)
docs: Create a concrete version of the substring
      :param start_idx : starting index of the substring
      :param end_idx : last index of the substring
      :param initial_string
  
      :return : a concrete version of the substring