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diff --git a/mltt/core/cases.ML b/mltt/core/cases.ML
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+(*  Title:      cases.ML
+    Author:     Joshua Chen
+
+Case reasoning.
+*)
+
+structure Case: sig
+
+val rules: Proof.context -> thm list
+val lookup_rule: Proof.context -> Termtab.key -> thm option
+val register_rule: thm -> Context.generic -> Context.generic
+
+end = struct
+
+(* Context data *)
+
+(*Stores elimination rules together with a list of the indexnames of the
+  variables each rule eliminates. Keyed by head of the type being eliminated.*)
+structure Rules = Generic_Data (
+  type T = thm Termtab.table
+  val empty = Termtab.empty
+  val extend = I
+  val merge = Termtab.merge Thm.eq_thm_prop
+)
+
+val rules = map #2 o Termtab.dest o Rules.get o Context.Proof
+fun lookup_rule ctxt = Termtab.lookup (Rules.get (Context.Proof ctxt))
+fun register_rule rl =
+  let val hd = Term.head_of (Lib.type_of_typing (Thm.major_prem_of rl))
+  in Rules.map (Termtab.update (hd, rl)) end
+
+
+(* [cases] attribute *)
+val _ = Theory.setup (
+  Attrib.setup \<^binding>\<open>cases\<close>
+    (Scan.succeed (Thm.declaration_attribute register_rule))
+    ""
+  #> Global_Theory.add_thms_dynamic (\<^binding>\<open>cases\<close>, rules o Context.proof_of)
+)
+
+
+end