Multiset Definition in Egison 2.3

Multiset.egi

(define $Multiset
  (lambda [$a]
    (type
      {[,$val []
        {[$tgt (match [val tgt] [(Multiset a) (Multiset a)]
                 {[[<nil> <nil>] {[]}]
                  [[<cons $x $xs> <cons ,x ,xs>] {[]}]
                  [[_ _] {}]})]}]
       [<nil> []
        {[{} {[]}]
         [_ {}]}]
       [<cons ,$px _> [(Multiset a)]
        {[$tgt {((remove a) tgt px)}]}]
       [<cons _ _> [a (Multiset a)]
        {[$tgt (map (lambda [$t] [t ((remove a) tgt t)])
                    ((unique a) tgt))]}]
       [<join ,$pxs _> [(Multiset a)]
        {[$tgt {((remove-collection a) tgt pxs)}]}]
       [<join _ _> [(Multiset a) (Multiset a)]
        {[$tgt (map (lambda [$ts] [ts ((remove-collection a) tgt ts)])
                    (subcollections tgt))]}]
       [_ [Something]
        {[$tgt {tgt}]}]
       })))