docs: def anti_commutator(A, B=None):
      """If ``B != None``, return the anti-commutator :math:`\{A,B\}`, otherwise
      return the super-operator :math:`\{A,\cdot\}`.  The super-operator
      :math:`\{A,\cdot\}` maps any other operator ``B`` to the anti-commutator
      :math:`\{A, B\} = A B + B A`.
  
      Args:
          A: The first operator to form all anti-commutators of.
          B: The second operator to form the anti-commutator of, or None.
  
      Returns:
          SuperOperator: The linear superoperator :math:`[A,\cdot]`
  
      """
      if B:
          return A * B + B * A
      return SPre(A) + SPost(A)