@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		@SuppressWarnings("rawtypes")
		ImmutableStack other = (ImmutableStack) obj;
		if (head == null) {
			if (other.head != null)
				return false;
		} else if (!head.equals(other.head))
			return false;
		if (tail == null) {
			if (other.tail != null)
				return false;
		} else if (!tail.equals(other.tail))
			return false;
		return true;
	}