% permutations: one list is a reordering of the items in the other list
% perm(-List1, +List2)
perm([],[]).
perm(X,[Y|Z]):-select(X,Y,R),perm(R,Z).
sudoku(Board):-Board=[R1,R2,R3,R4],
	Board=[[X11,X12,X13,X14],[X21,X22,X23,X24],[X31,X32,X33,X34],[X41,X42,X43,X44]],
	D=[1,2,3,4], %the digits
	X11=1, X22=2,X33=3,X44=4,
	perm(R1,D), perm(R2,D), perm(R3,D),perm(R4,D),
	perm([X11,X21,X31,X41],D), %Columns
	perm([X12,X22,X32,X42],D),
	perm([X13,X23,X33,X43],D),
	perm([X14,X24,X34,X44],D),
	perm([X11,X12,X21,X22],D), %Boxes
	perm([X13,X14,X23,X24],D),
	perm([X31,X32,X41,X42],D),
	perm([X33,X34,X43,X44],D),
	write_ln(R1),
	write_ln(R2),
	write_ln(R3),
	write_ln(R4).