undo for initial metrical instruction

engine/api.py

@@ -1955,7 +1955,17 @@ def insertMetricalInstruction(treeOfInstructions, lilypondOverride = None):
 
 
 def setInitialMetricalInstruction(treeOfInstructions, lilypondOverride:str=None):
-    """Variation of insertMetricalInstruction to set the initial one for ALL tracks"""
+    """Variation of insertMetricalInstruction to set the initial one for ALL tracks.
+    Undo depends on it that all tracks have the same initial."""
+
+    #Undo
+    oldInitial = session.data.tracks[0].initialMetricalInstruction #the first one is the same as all.
+    def registeredUndoFunction():
+        setInitialMetricalInstruction(oldInitial.treeOfInstructions, oldInitial.lilypondParameters["override"])
+    session.history.register(registeredUndoFunction, descriptionString=f"Initial Metrical Instruction")
+    callbacks._historyChanged()
+
+    #New one
     item = items.MetricalInstruction(treeOfInstructions)
 
     if lilypondOverride: