Arities cons 2, tail 1, nil 0, filter 1, 0 0, 1 0, failure 1.

# filter records duplicate elements of a list, e.g.
# filter([1,1,0,1,0,0]) = [1,0]

filter(nil) => nil .
filter(cons(X,nil)) => nil .
filter(cons(X,cons(X,Xs))) => cons(X,filter(cons(X,Xs))) .
filter(cons(0,cons(1,Xs))) => filter(cons(1,Xs)) .
filter(cons(1,cons(0,Xs))) => filter(cons(0,Xs)) .


# Usual tail function.
tail(cons(X,Xs)) => Xs .

# Unwanted prefix: 01
cons(0,cons(1,X)) => failure(cons(0,cons(1,X))) .

| @c : failure(X) => @zzResult : !FAIL! .

%%

# Take the language (10|0)* ...
@a : nil 
   , cons(1,cons(0,@a))
   , cons(0,@a) .

# ... compute its suffixes ...
@b : @a
   , tail(@b) .

# ... and filter them.
@c : filter(@b) .