For handling subgrids the following function may be useful:

• given a sudoku grid of size m * m containing integers,
• the function returns a list of length m,
• whose elements are lists of integers also of length m,
• each of which contains the elements of a subgrid
• (and all subgrids are included).

For now I just give these hints:

1. It is possible to write a Prolog program for this task without any recursive predicates, making use of the following helper predicates:
   • transpose/2 from library(lists)
   • append/2 from library(lists)
   • maplist/3 from library(lists)
   • chop/3 from homework 7

   If you are using SWI Prolog, you can load the transpose from this file, or simply use :- library(clpfd).

2. It is also possible to solve the task in such a way that chop/3 is called exactly twice.

You may find useful some predicates in SICStus Prolog library(lists), e.g. transpose/2 and sublist/5. In solving this assignment you can freely use predicates from this library.

In SWI Prolog transpose/2 is part of library(clpfd). You can load this library for the purpose of using transpose/2, but no other predicates should be used from library(clpfd). Also, sublist/5 is not available in SWI, you can use list_sub/5 from Homework P4 instead.

Ask the instructors if you would like to use other libraries.

Some predicates you wrote for earlier homework may be re-used here, such as chop/3 from Homework P4, and pairings/2 from Homework P4.

Please send an email if you need further clarification or help.

Last modified by Péter Szeredi, szeredi@cs.bme.hu, on 11.11.2018.