True when the tags of Select and From have been
unified, all keys in Select appear in From and the
corresponding values have been unified. The key-value pairs of From
that do not appear in Select are used to form an anonymous
dict, which us unified with Rest. For example:
?- select_dict(P{x:0, y:Y}, point{x:0, y:1, z:2}, R).
P = point,
Y = 1,
R = _G1705{z:2}.
See also :</2 to
ignore Rest and >:</2
for a symmetric partial unification of two dicts.