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+/* MONEY, a crypto-arithmetic puzzle */
+
+/* Written in GNU MathProg by Andrew Makhorin <mao@gnu.org> */
+
+/* This is the classic example of a crypto-arithmetic puzzle published
+   in the Strand Magazine by Henry Dudeney:
+
+        S E N D
+      +
+        M O R E
+      ---------
+      M O N E Y
+
+   In this puzzle the same letters mean the same digits. The question
+   is: how to replace all the letters with the respective digits that
+   makes the calculation correct?
+
+   The solution to this puzzle is:
+   O = 0, M = 1, Y = 2, E = 5, N = 6, D = 7, R = 8, and S = 9.
+
+   References:
+   H. E. Dudeney, in Strand Magazine vol. 68 (July 1924), pp. 97, 214.
+
+   (From Wikipedia, the free encyclopedia.) */
+
+set LETTERS := { 'D', 'E', 'M', 'N', 'O', 'R', 'S', 'Y' };
+/* set of letters */
+
+set DIGITS := 0..9;
+/* set of digits */
+
+var x{i in LETTERS, d in DIGITS}, binary;
+/* x[i,d] = 1 means that letter i is digit d */
+
+s.t. one{i in LETTERS}: sum{d in DIGITS} x[i,d] = 1;
+/* each letter must correspond exactly to one digit */
+
+s.t. alldiff{d in DIGITS}: sum{i in LETTERS} x[i,d] <= 1;
+/* different letters must correspond to different digits; note that
+   some digits may not correspond to any letters at all */
+
+var dig{i in LETTERS};
+/* dig[i] is a digit corresponding to letter i */
+
+s.t. map{i in LETTERS}: dig[i] = sum{d in DIGITS} d * x[i,d];
+
+var carry{1..3}, binary;
+/* carry bits */
+
+s.t. sum1: dig['D'] + dig['E']            = dig['Y'] + 10 * carry[1];
+s.t. sum2: dig['N'] + dig['R'] + carry[1] = dig['E'] + 10 * carry[2];
+s.t. sum3: dig['E'] + dig['O'] + carry[2] = dig['N'] + 10 * carry[3];
+s.t. sum4: dig['S'] + dig['M'] + carry[3] = dig['O'] + 10 * dig['M'];
+s.t. note: dig['M'] >= 1; /* M must not be 0 */
+
+solve;
+/* solve the puzzle */
+
+display dig;
+/* and display its solution */
+
+end;