src/Pure/System/isabelle_system.scala

   File, IOException}
 import java.awt.{GraphicsEnvironment, Font}
 
 import scala.io.Source
 import scala.util.matching.Regex
+import scala.collection.mutable
 
 
 object Isabelle_System
 {
   val charset = "UTF-8"

   /* find logics */
 
   def find_logics(): List[String] =
   {
     val ml_ident = getenv_strict("ML_IDENTIFIER")
-    var logics: Set[String] = Set()
+    val logics = new mutable.ListBuffer[String]
     for (dir <- getenv_strict("ISABELLE_PATH").split(":")) {
       val files = platform_file(dir + "/" + ml_ident).listFiles()
       if (files != null) {
         for (file <- files if file.isFile) logics += file.getName
       }