Eight Queens

The Problem

Place eight queens on a chessboard so that none is captured by any other. A placement is represented as a permutation of !8:
	p:2 5 3 0 7 4 6 1
p[0] says that a queen is placed in column 2 of row 0. p[1] says that a queen is placed in column 5 of row 1. : A permutation is displayed with the 'board' function 'bd':
Horizontal and vertical captures are not possible on a board represented by a permutation.

The Algorithm

        place a queen in column j of row 0
        in row 1, mark:
            vertical column j
            left diagonal column j-1
            right diagonal column j+1
        place a queen in an unmarked column k of row 1
        in row 2, mark:
            vertical columns j, k
            left diagonal columns j-2, k-1
            right diagonal colunms j+2, k+1

The Code (generalized for n queens)