1 files changed, 1 insertions, 1 deletions
diff --git a/isa/isa.el b/isa/isa.el
index 9482e3dc..c4f58f25 100644
--- a/isa/isa.el
+++ b/isa/isa.el
@@ -546,7 +546,7 @@ Resulting output from Isabelle will be parsed by Proof General."
 
 (defun isa-state-preserving-p (cmd)
   "Non-nil if command preserves the proofstate."
-  (proof-string-match isa-not-undoable-commands-regexp cmd))
+  (not (proof-string-match isa-not-undoable-commands-regexp cmd)))
 
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