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diff --git a/lib/schema.ml b/lib/schema.ml
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+(** Schema: maps attribute names to domain/type names.
+
+    A schema defines the structure of a relation by listing each attribute
+    and its associated domain (type). This is used for:
+    - Hashing relations (schema is part of the relation identity)
+    - Validation (checking tuple membership)
+    - Persistence (storing and loading relations with their structure) *)
+
+type t = (string * string) list
+
+let empty : t = []
+
+let add name domain_name (schema : t) : t =
+  (name, domain_name) :: schema
+
+let attributes (schema : t) =
+  List.map fst schema
+
+let to_string schema =
+  List.map (fun (n, t) -> n ^ ":" ^ t) schema
+  |> String.concat ","