Proof General -- if present make this the default;

--- a/etc/settings	Wed May 17 17:16:21 2000 +0200
+++ b/etc/settings	Wed May 17 18:27:13 2000 +0200
@@ -135,8 +135,10 @@
 ISAMODE_HOME=$ISABELLE_HOME/contrib/Isamode
 ISAMODE_OPTIONS=""
 
-# Emacs running Proof General.
-#ISABELLE_INTERFACE=$ISABELLE_HOME/contrib/ProofGeneral/isar/interface
+# Proof General -- if present make this the default
+if [ -d "$ISABELLE_HOME/contrib/ProofGeneral" ]; then
+  ISABELLE_INTERFACE=$ISABELLE_HOME/contrib/ProofGeneral/isar/interface
+fi
 PROOFGENERAL_OPTIONS=""