Here's the recipe: given a starting infostate s, choose an object a
from the domain of discourse. Construct a modified infostate s' by
-adjusting the assignment function in order to map the variable x to a.
+adjusting the assignment function of each possibility so as to map the variable x to a.
Then update s' with φ. Finally, take the union over the results of
doing this for every object a in the domain of discourse.