class_docstr: Represents positions in a target sentence. Used to keep track of
  which slot (position) is occupied.
docstr: :return: Number of vacant slots up to, and including, ``position``
docstr_tok: [":","return",":","Number","of","vacant","slots","up","to",",","and","including",",","``","position","``"]
code: def vacancies_at(self, position):
      """
      :return: Number of vacant slots up to, and including, ``position``
      """
      vacancies = 0
      for k in range(1, position + 1):
          if not self._slots[k]:
              vacancies += 1
      return vacancies
  
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 3 for calling the Slots obj's underlying member method code with example usage: obj.vacancies_at(position) and returns: vacancies