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t2:=("a","a","a","e","e","i","i","i","u") [enter]
t2.count(t2,"a") [enter] returns 1? > 3

t2 is a sequence, therefore evaluation of t2,"a" will add "a" to the sequence.

This feature has not yet been updated.

It will never be, because of the nature of the objects. Sequences in Xcas are != from tuples in Python.