list - Prolog removing all variables and duplicates -

i need query, remove me variables , duplicates list.

example:

?- l = [1,2,3,x,y,3,2], my_awesome_predicate(l, res). 

then, res should be: [1,2,3].

i don't care order (it [2,3,1], [3,2,1] or whatever).

unfortunately, have task in have care efficiency, main question - can done faster? currently, have following code:

remove_variables([], []). remove_variables([h|list], res):- var(h), !, remove_variables(list, res). remove_variables([h|list], [h|res]):- remove_variables(list, res).  my_awesome_predicate([], []). my_awesome_predicate(list, res):-   sort(list, sorted),   remove_variables(sorted, res). 

if using swi can improve little further code:

my_awesome_predicate(list, res):-   sort(list, mres),   remove_variables(mres, res).  remove_variables([var|tail], ntail):-   var(var),   !,   remove_variables(tail, ntail). remove_variables(res, res). 

as seems swi's sort leave unbounded variables first (don't know if behavior standard among other prolog's), can stop removing variables once find first non-variable.

reading bit swi's documentation, it's stated that:

4.7.1 standard order of terms  comparison ,  unification of arbitrary  terms.   terms ordered  in called ``standard order''.  order defined follows:   1. variables < numbers < atoms < strings < compound terms 

so seems safe stop removing elements when find first non-variable...