Hi Mauricio,
My apologies for the late response! And congratulations for having made it that far. The part of your example that isn't working is the xforms-value-changed on the xf:group, because the xf:group doesn't really hold a value, so doesn't trigger an xforms-value-change. Things will work if you change it to xf:variable, which is very much like a control with no UI and that as a "side effect" declares a variable.
For reference, here the updated
input-test.xbl.
Alex
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