class_docstr: Represents positions in a target sentence. Used to keep track of
  which slot (position) is occupied.
code: def __len__(self):
      return len(self._slots) - 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 4 for calling the Slots obj's underlying member method code with example usage: obj.__len__() and returns: unknown