add queues, fix debray allocator bug.

src/prolog/allocator.rs

@@ -19,6 +19,7 @@ pub trait Allocator<'a>
     fn reset(&mut self);
     fn reset_contents(&mut self) {}
     fn reset_arg(&mut self, usize);
+    fn reset_at_head(&mut self, &Vec<Box<Term>>);
     
     fn advance_arg(&mut self);