A.9.17.4 Global constraints
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Availability::- use_module(library(clpfd)).
Sourcechain(+Zs, +Relation)
Zs form a chain with respect to Relation. Zs is a list of finite domain variables that are a chain with respect to the partial order Relation, in the order they appear in the list. Relation must be #=, #=<, #>=, #< or #>. For example: 
?- chain([X,Y,Z], #>=).
X#>=Y,
Y#>=Z.