Nondeterministically instantiates a domain variable to its domain values. The order of enumeration is in increasing term order (i.e. alphabetic for atoms).
    ?- X &:: [mo, tu, we, th, fr, sa, su], indomain(X).
    X = fr
    Yes (0.00s cpu, solution 1, maybe more)
    X = mo
    Yes (0.05s cpu, solution 2, maybe more)
    X = sa
    Yes (0.06s cpu, solution 3, maybe more)
    X = su
    Yes (0.06s cpu, solution 4, maybe more)
    X = th
    Yes (0.06s cpu, solution 5, maybe more)
    X = tu
    Yes (0.07s cpu, solution 6, maybe more)
    X = we
    Yes (0.08s cpu, solution 7)
    ?- indomain(we).
    Yes (0.00s cpu)