description: common[op] sp find kets in common, with respect to an operator examples: friends |Fred> => |Jack> + |Harry> + |Ed> + |Mary> + |Rob> + |Patrick> + |Emma> + |Charlie> friends |Sam> => |Charlie> + |George> + |Emma> + |Jack> + |Rober> + |Frank> + |Julie> common[friends] split |Fred Sam> |Jack> + |Emma> + |Charlie> see also: union