Contains

Returns true when any element of `haystack` equals `needle`, using the same semantics as `tx.Equals` (URI equality for refs/subjects, value equality for literals, cross-kind false). Equivalent to a `tx.Filter` (over haystack with `tx.Equals` predicate) followed by `tx.Count > 0`, but spares the author the boilerplate. Reach for it constantly in diff / lookup workflows: "is this thesis in the previous snapshot's hasThesis list?"