src/Pure/Build/build.scala

   /* context */
 
   sealed case class Context(
     store: Store,
     build_deps: isabelle.Sessions.Deps,
-    engine: Engine = Default_Engine,
+    engine: Engine = Engine.Default,
     afp_root: Option[Path] = None,
     build_hosts: List[Build_Cluster.Host] = Nil,
     ml_platform: String = Isabelle_System.getenv("ML_PLATFORM"),
     hostname: String = Isabelle_System.hostname(),
     numa_shuffling: Boolean = false,

     lazy val services: List[Engine] =
       Isabelle_System.make_services(classOf[Engine])
 
     def apply(name: String): Engine =
       services.find(_.name == name).getOrElse(error("Bad build engine " + quote(name)))
+
+    class Default extends Engine("") { override def toString: String = "<default>" }
+    object Default extends Default
   }
 
   class Engine(val name: String) extends Isabelle_System.Service {
     override def toString: String = name
 

         using(open_build_process(context, progress, server))(_.run())
       }
     }
   }
 
-  class Default_Engine extends Engine("") { override def toString: String = "<default>" }
-  object Default_Engine extends Default_Engine
 
 
   /* build */
 
   def build(