The u's and x's are syntactically quite similar. Semantically, they are
totally different, though. Whereas an "x" creates a list of dates, an "u"
introduces an unspecified digit.
The difference is important when handling an L2Interval. Whereas
makes sense, I consider that
does not make sense.
In other words, I suggest to rewrite the section on masked dates to keep
x's and u's apart.