(run* [q]
  (nom/fresh [a b c d]
    (fresh [x y z]
      (== (nom/tie a (nom/tie b y)) (nom/tie b (nom/tie a x)))
      (== (nom/tie c (nom/tie d x)) (nom/tie d (nom/tie c z)))
      (== x y)
      (== z x))))

The snippet above fails with a no implementation of method -merge-doms for LazySeq, because -merge-doms for ::nom is implemented for vectors but returns a lazy seq, not a vector.

The simplest fix is to change `concat` to `into` in the -merge-doms implementation of nominal.clj. However, while I am at it, I will replace the vector with a set, which makes more sense given how it's used anyways.

Patch will be attached shortly.