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Let us return briefly to the Send More Money problem
(see Constraint Satisfaction Problems). Its sum/8 predicate
will expand to a scalar_product/4 constraint. An indexical
version is defined simply by changing the neck symbol of sum/8
from ‘:-’ to ‘+:’, thus turning it into an FD predicate:
sum(S, E, N, D, M, O, R, Y) +:
1000*S + 100*E + 10*N + D
+ 1000*M + 100*O + 10*R + E
#= 10000*M + 1000*O + 100*N + 10*E + Y.