fn alloc_one<T>(&mut self) -> Result<Unique<T>, AllocErr>
        where Self: Sized
    {
        let k = Layout::new::<T>();
        if k.size() > 0 {
            unsafe { self.alloc(k).map(|p|Unique::new(*p as *mut T)) }
        } else {
            Err(AllocErr::invalid_input("zero-sized type invalid for alloc_one"))
        }
    }