diff --git a/TeXmacs/progs/dynamic/session-edit.scm b/TeXmacs/progs/dynamic/session-edit.scm
index 04cdb1cff2..6c16671c3b 100644
--- a/TeXmacs/progs/dynamic/session-edit.scm
+++ b/TeXmacs/progs/dynamic/session-edit.scm
@@ -61,6 +61,9 @@
                 (cons (car opts) (cadr opts)))
     (ahash-ref session-math-input key)))
 
+(tm-define (session-enable-math-input lan ses)
+  (ahash-set! session-math-input (cons lan ses) #t))
+
 (tm-define (toggle-session-math-input)
   (:synopsis "Toggle mathematical input in sessions.")
   (:check-mark "v" session-math-input?)