public String toString()
	{
		switch(this.typeid)
		{
		case t_error:
		case t_int:
		case t_bool:
		case t_unit:
		case t_nil:
		case t_function:
			return this.typeid.toString();
		case t_pair:
			return this.t1.toString()+" * " +this.t2.toString();
		case t_list:
			return this.t1.toString()+" list";
		}
		return "";
	}