code: def apply(self, flag_set: AbstractSet[Flag], operand: AbstractSet[Flag]) \
              -> FrozenSet[Flag]:
          
          if self == FlagOp.ADD:
              return frozenset(flag_set | operand)
          elif self == FlagOp.DELETE:
              return frozenset(flag_set - operand)
          else:  # op == FlagOp.REPLACE
              return frozenset(operand)
docs: Apply the flag operation on the two sets, returning the result.
  
          Args:
              flag_set: The flag set being operated on.
              operand: The flags to use as the operand.