Added N-Queens example

adapted from tests

nqueens.f

@@ -0,0 +1,33 @@
+\ Adapted from the forth version at 
+\ http://rosettacode.org/wiki/N-queens_problem
+\ requires jonesforth.f
+
+VARIABLE SOLUTIONS
+VARIABLE NODES
+
+: BITS ( N -- MASK ) 1 SWAP LSHIFT 1- ;
+: LOWBIT  ( MASK -- BIT ) DUP NEGATE AND ;
+: LOWBIT- ( MASK -- BITS ) DUP 1- AND ;
+
+: NEXT3 ( DL DR F FILES -- DL DR F DL' DR' F' )
+  INVERT >R
+  2 PICK RSP@ @ AND 2* 1+
+  2 PICK RSP@ @ AND 2/
+  2 PICK R> AND ;
+
+: TRY ( DL DR F -- )
+  DUP IF
+    1 NODES +!
+    DUP 2OVER AND AND
+    BEGIN ?DUP WHILE
+      DUP >R LOWBIT NEXT3 RECURSE R> LOWBIT-
+    REPEAT
+  ELSE 1 SOLUTIONS +! THEN
+  DROP 2DROP ;
+
+: QUEENS ( N -- )
+  0 SOLUTIONS ! 0 NODES !
+  -1 -1 ROT BITS TRY
+  SOLUTIONS @ . ." SOLUTIONS, " NODES @ . ." NODES" CR ;
+
+8 QUEENS  \ 92 SOLUTIONS, 1965 NODES