Let us return briefly to the Send More Money problem (see A Constraint Satisfaction Problem). Its sum/8 predicate
will expand to a space-efficient conjunction of library
constraints. A faster but more memory consuming 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.