docs: def filter_cid(cls, ops, kwargs):
      """Remove occurrences of the :func:`.circuit_identity` ``cid(n)`` for any
      ``n``. Cf. :func:`filter_neutral`
      """
      from qnet.algebra.core.circuit_algebra import CircuitZero, circuit_identity
      if len(ops) == 0:
          return CircuitZero
      fops = [op for op in ops if op != circuit_identity(op.cdim)]
      if len(fops) > 1:
          return fops, kwargs
      elif len(fops) == 1:
          # the remaining operand is the single non-trivial one
          return fops[0]
      else:
          # the original list of operands consists only of neutral elements
          return ops[0]