* N queens problem, a string oriented version to
*   demonstrate the power of pattern matching.
* A numerically oriented version will run faster than this.
* Program written by Viktors Berstis
        N = 5
        NM1 = N - 1; NP1 = N + 1; NSZ = N * NP1; &STLIMIT = 10 ** 9; &ANCHOR = 1
        DEFINE('SOLVE(B)I')
* This pattern tests if the first queen attacks any of the others:
        TEST = BREAK('Q') 'Q' (ARBNO(LEN(N) '-') LEN(N) 'Q'
+             | ARBNO(LEN(NP1) '-') LEN(NP1) 'Q'
+             | ARBNO(LEN(NM1) '-') LEN(NM1) 'Q')
        P = LEN(NM1) . X LEN(1); L = 'Q' DUPL('-',NM1) ' '
        SOLVE()        :(END)
SOLVE   EQ(SIZE(B),NSZ)             :S(PRINT)
* Add another row with a queen:
        B = L B
LOOP    I = LT(I,N) I + 1 :F(RETURN)
        B TEST :S(NEXT)
        SOLVE(B)
* Try queen in next square:
NEXT    B P = '-' X :(LOOP)
PRINT   SOLUTION = SOLUTION + 1
        OUTPUT = 'Solution number ' SOLUTION ' is:'
PRTLOOP B LEN(NP1) . OUTPUT = :S(PRTLOOP)F(RETURN)
END