core-logic

2016-04-09T00:14:39.000072Z

I am not entirely sure, but I suspect the issue is the lvar not being grounded in your second set of examples, because the lvar isn't bound to a definite value it is sort of undecided either way

2016-04-09T00:15:17.000073Z

I think to do what you want, instead of using the lvar like that, you want to say none of the elements of the sequence have a first value that is ':b'