?- symbol_domain_index(fr, D, I).
D = weekday
I = 5
Yes (0.00s cpu)
?- X &:: weekday, symbol_domain_index(X, D, I).
X = X{[mo, tu, we, th, fr, sa, su]}
D = weekday
I = I{1 .. 7}
Yes (0.00s cpu)
?- X &:: weekday, X &\= we, symbol_domain_index(X, D, I).
X = X{[mo, tu, th, fr, sa, su]}
D = weekday
I = I{[1, 2, 4 .. 7]}
Yes (0.00s cpu)
The integer variable I mirrors the domain of the symbolic variable X and vice versa.