Premise: Postal codes in Brunei are alphanumeric, consisting of two letters followed by four digits in the format of YZ0000, where Y denotes the district code, Z denotes the mukim code, the first two digits denote the area or village code, and the last two digits denote the nearest post office code (e.g. the postal code for Pantai Mentiri Golf Club is BU2529).
Hypothesis: The district code is always a number