src/Pure/General/path.scala

     def eval(elem: Path.Elem): List[Path.Elem] =
       elem match {
         case Path.Variable(s) =>
           val path = Path.explode(Isabelle_System.getenv_strict(s, env))
           if (path.elems.exists(_.isInstanceOf[Path.Variable]))
-            error("Illegal path variable nesting: " + Properties.Eq(s -> path.toString))
+            error("Illegal path variable nesting: " + Properties.Eq(s, path.toString))
           else path.elems
         case x => List(x)
       }
 
     new Path(Path.norm_elems(elems.flatMap(eval)))