tuned -- accommodate scala-2.13.0-RC3;

--- a/src/Pure/PIDE/prover.scala	Sat Jun 01 13:53:23 2019 +0200
+++ b/src/Pure/PIDE/prover.scala	Sat Jun 01 21:43:03 2019 +0200
@@ -231,7 +231,7 @@
           }
           if (result.length > 0) {
             output(markup, Nil, List(XML.Text(Symbol.decode(result.toString))))
-            result.length = 0
+            result.clear
           }
           else {
             reader.close

--- a/src/Pure/System/tty_loop.scala	Sat Jun 01 13:53:23 2019 +0200
+++ b/src/Pure/System/tty_loop.scala	Sat Jun 01 21:43:03 2019 +0200
@@ -29,7 +29,7 @@
         if (result.length > 0) {
           System.out.print(result.toString)
           System.out.flush()
-          result.length = 0
+          result.clear
         }
         else {
           reader.close()