13. Pose and solve the crypt-arithmetic problem SEND + MORE = MONEY as a CSP. In a cryptarithmetic problem, each letter represents a different digit, the leftmost digit cannot be zero

(because then it would not be there), and the sum must be correct considering each sequence of letters as a base ten numeral. In this example, you know that Y = (D + E) mod 10 and that E = (N + R + ((D + E) ÷ 10)) mod 10, and so on.