: nil? nil = ;
: not-nil? nil? invert ;
: cons ( tail, head -- consc )  , , HERE 2 cells - ;
: head ( consc -- e )  @ ;
: tail ( consc -- e )  1 cells + @ ;
: even? ( n -- bool ) 2 mod 0 = ;
: odd? ( n -- bool ) even? invert ;

nil 1 cons head 1 = .
nil 1 cons tail nil = .
nil 2 cons 1 cons head 1 = .
nil 2 cons 1 cons tail head 2 = .
nil 2 cons 1 cons tail tail nil = .
nil 3 cons 2 cons 1 cons head 1 = .

nil 3 cons 2 cons 1 cons CONSTANT alist

alist head 1 = .
alist tail head 2 = .

2 even? .
3 odd? .

: filter ( pred, consc -- consc ) recursive
	2dup not-nil? if
		over dup head rot execute if
			tail swap head cons filter
		else
			drop tail filter
		then
	else
		drop nip
	then ;

' odd? alist filter