Initial data commit

Task/N-queens-problem/Pure/n-queens-problem.pure

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+/*
+ n-queens.pure
+ Tectonics:
+     pure -c queens.pure -o queens
+  or
+     pure -q -i queens.pure
+*/
+using system;
+
+queens n = search n 1 [] with
+   search n i p  = [reverse p] if i>n;
+                 = cat [search n (i+1) ((i,j):p) | j = 1..n; safe (i,j) p];
+   safe (i,j) p  = ~any (check (i,j)) p;
+   check (i1,j1) (i2,j2)
+                 = i1==i2 || j1==j2 || i1+j1==i2+j2 || i1-j1==i2-j2;
+end;
+
+compiling || (puts("queens 4: " + str(queens 4)) $$
+              puts("Solutions to queens 7: " + str(#queens 7)));