Availability:
:- use_module(library(clpfd)).
sum(+Vars, +Rel,
?Expr)\=, #<, #>, #=<
or #>=. For example:
?- [A,B,C] ins 0..sup, sum([A,B,C], #=, 100). A in 0..100, A+B+C#=100, B in 0..100, C in 0..100.
sum(+Vars, +Rel,
?Expr)