old_contents: """
  A collection of update operations for TinyDB.
  
  They are used for updates like this:
  
  >>> db.update(delete('foo'), where('foo') == 2
  
  This would delete the ``foo`` field from all documents where ``foo`` equals 2.
  """
  
  
  def delete(field):
      """
      Delete a given field from the document.
      """
      def transform(doc):
          del doc[field]
  
      return transform
  
  
  def add(field, n):
      """
      Add ``n`` to a given field in the document.
      """
      def transform(doc):
          doc[field] += n
  
      return transform
  
  
  def subtract(field, n):
      """
      Substract ``n`` to a given field in the document.
      """
      def transform(doc):
          doc[field] -= n
  
      return transform
  
  
  def set(field, val):
      """
      Set a given field to ``val``.
      """
      def transform(doc):
          doc[field] = val
  
      return transform
  
  
  def increment(field):
      """
      Increment a given field in the document by 1.
      """
      def transform(doc):
          doc[field] += 1
  
      return transform
  
  
  def decrement(field):
      """
      Decrement a given field in the document by 1.
      """
      def transform(doc):
          doc[field] -= 1
  
      return transform
  
new_contents: """
  A collection of update operations for TinyDB.
  
  They are used for updates like this:
  
  >>> db.update(delete('foo'), where('foo') == 2)
  
  This would delete the ``foo`` field from all documents where ``foo`` equals 2.
  """
  
  
  def delete(field):
      """
      Delete a given field from the document.
      """
      def transform(doc):
          del doc[field]
  
      return transform
  
  
  def add(field, n):
      """
      Add ``n`` to a given field in the document.
      """
      def transform(doc):
          doc[field] += n
  
      return transform
  
  
  def subtract(field, n):
      """
      Substract ``n`` to a given field in the document.
      """
      def transform(doc):
          doc[field] -= n
  
      return transform
  
  
  def set(field, val):
      """
      Set a given field to ``val``.
      """
      def transform(doc):
          doc[field] = val
  
      return transform
  
  
  def increment(field):
      """
      Increment a given field in the document by 1.
      """
      def transform(doc):
          doc[field] += 1
  
      return transform
  
  
  def decrement(field):
      """
      Decrement a given field in the document by 1.
      """
      def transform(doc):
          doc[field] -= 1
  
      return transform
  
subject: fix typo in tinydb.operations.py doc string (#337)