src/Pure/System/build.scala

       val option =
         name ~ opt(keyword("=") ~! name ^^ { case _ ~ x => x }) ^^ { case x ~ y => (x, y) }
       val options = keyword("[") ~> repsep(option, keyword(",")) <~ keyword("]")
 
       val theories =
-        keyword(THEORIES) ~! ((options | success(Nil)) ~ rep1(theory_name)) ^^
+        keyword(THEORIES) ~! ((options | success(Nil)) ~ rep(theory_name)) ^^
           { case _ ~ (x ~ y) => (x, y) }
 
       ((command(SESSION) ~! session_name) ^^ { case _ ~ x => x }) ~
         (keyword("(") ~! (rep1(name) <~ keyword(")")) ^^ { case _ ~ x => x } | success(Nil)) ~
         (keyword(IN) ~! path ^^ { case _ ~ x => x } | success(".")) ~