Question: indets is different in this case, how to do?

indets(AA(MM(AA(X1,X2),B2),MM(AA(B2,X3),X5)));
this can get a list of X1,X2,B2,X3,X5

propositionsentence := MM(AA("it", "run"), "maple"):
indets(propositionsentence);

but this can not get "it", "run", "maple"

how to do this in case this in maple 12?
 

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