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I have found the following behavior of Ci and Si without arguments:

In HOME:
Ci() returns: -Inf
Si() returns: []

In CAS:
Ci() returns: -∞
Si() returns: []

So, it seems that Ci() is using 0 as the default argument, but does anyone know the meaning of [] for Si() ?

UPDATE.
I have found that Ei also has an odd behavior when no arguments are supplied:

In HOME and CAS:
Ei() returns: Ei

In the new FW version 2.1.14181 (2018 10 16) all three functions Ci(), Si() and Ei(), return [] both in HOME and CAS. This looks more coherent than the previous behavior.