In the default execution mode, CLP(FD) constraints still exhibit some
non-relational properties. For example, adding constraints can
yield new solutions:
?- X #= 2, X = 1+1.
false.
?- X = 1+1, X #= 2, X = 1+1.
X = 1+1.
This behaviour is highly problematic from a logical point of view,
and it may render declarative debugging techniques inapplicable.
Set the Prolog flag clpfd_monotonic to true to
make CLP(FD)
monotonic: This means that adding new constraints cannot
yield new solutions. When this flag is true, we must wrap
variables that occur in arithmetic expressions with the functor (?)/1
or (#)/1. For example:
?- set_prolog_flag(clpfd_monotonic, true).
true.
?- #(X) #= #(Y) + #(Z).
#(Y)+ #(Z)#= #(X).
?- X #= 2, X = 1+1.
ERROR: Arguments are not sufficiently instantiated
The wrapper can be omitted for variables that are already constrained
to integers.
