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)