Stupid U-label question

[Frank Ellermann]

Hi, triggered by an error - I tried a punycode decode
instead of an IDNA2003 decode - I tested the effect
of xn--xn--4ca-cxa with two tools, both told me "this
is no A-label".

In other words xn--4caä (ending with an umlauted a)
is no U-label.  I wasn't aware that U-labels must not
start with "xn--", is that mentioned in the drafts ?

 Frank