Ambiguity with cons null and disequality constraint

[brandonwillard]

This isn't a problem in the neq constraint, but it's a confusing ambiguity/gotcha that arises from cons's "null" value for lists being both [] and None (i.e. cons(1, []) == [1] and cons(1, None) == [1]).

This simple examples shows how the ambiguity can arise.

from kanren import run, eq, conde, lall, var
from kanren.constraints import neq
from cons import cons

a_lv, b_lv = var(), var()
res = run(0, a_lv,
          neq([1], cons(1, a_lv)),
          eq(a_lv, b_lv),
          eq(b_lv, None))

assert res == ()

The assertion fails, even though the cons(1, a_lv) term will reify to [1].