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