class_docstr: An abstract class that takes a sentence and transforms it by replacing
  some of its words.
  
  letters_to_insert (string): letters allowed for insertion into words
  (used by some char-based transformations)
code: def __init__(self, letters_to_insert=None):
      self.letters_to_insert = letters_to_insert
      if not self.letters_to_insert:
          self.letters_to_insert = string.ascii_letters
  
file_path: TextAttack/textattack/transformations/word_swaps/word_swap.py
imports: import random
  import string
  from textattack.transformations import Transformation
label_desc: Use image node_id 1 to create a new WordSwap object from inherited base classes: Transformation with example: obj = WordSwap(letters_to_insert)