public String toString()
	{
	    String return_str = "";
	    switch(num_action)
		{
		case 1: return_str = "node  creat " + n; break;
		case 2: return_str = "node  suppr " + n; break;
		case 3: return_str = "trace creat " + t; break;
		case 4: return_str = "trace suppr " + t; break;
		case 5: return_str = "node  edit  " + n + " Ls:" + last_start + ", " + "Le:" + last_end + ", " + "Lv:" + last_value; break;
		case 6: return_str = "trace edit  " + t; break;
		case 7: return_str = "node  move  " + n + " (" + old_pos_x + ", " + old_pos_y + ")"; break; 
		}
	    return num_action + ":= " + return_str;
	}