class_docstr: Represents positions in a target sentence. Used to keep track of
  which slot (position) is occupied.
code: def __init__(self, target_sentence_length):
      self._slots = [False] * (target_sentence_length + 1)
  
imports: import warnings
  from collections import defaultdict
  from math import factorial
  from nltk.translate import AlignedSent, Alignment, IBMModel, IBMModel4
  from nltk.translate.ibm_model import Counts, longest_target_sentence_length
label_desc: Use image node_id 1 to create a new Slots object  with example: obj = Slots(target_sentence_length)