src/HOL/Nominal/nominal_induct.ML

 val ruleN = "rule";
 
 val inst = Scan.lift (Args.$$$ "_") >> K NONE || Args.term >> SOME;
 
 val def_inst =
-  ((Scan.lift (Args.name --| (Args.$$$ "\\<equiv>" || Args.$$$ "==")) >> SOME)
+  ((Scan.lift (Args.name --| (Args.$$$ "\<equiv>" || Args.$$$ "==")) >> SOME)
       -- Args.term) >> SOME ||
     inst >> Option.map (pair NONE);
 
 val free = Scan.state -- Args.term >> (fn (_, Free v) => v | (ctxt, t) =>
   error ("Bad free variable: " ^ Syntax.string_of_term (Context.proof_of ctxt) t));