1.1 --- a/src/Pure/System/session.scala	Tue Aug 30 15:49:27 2011 +0200
     1.2 +++ b/src/Pure/System/session.scala	Tue Aug 30 16:04:26 2011 +0200
     1.3 @@ -243,7 +243,7 @@
     1.4  
     1.5        def id_command(command: Command)
     1.6        {
     1.7 -        if (global_state().lookup_command(command.id).isEmpty) {
     1.8 +        if (!global_state().defined_command(command.id)) {
     1.9            global_state.change(_.define_command(command))
    1.10            prover.get.define_command(command.id, Symbol.encode(command.source))
    1.11          }