On Thu, 1 Feb 2018 01:38:58 +0000 Richard Wordingham via Unicode <unicode@unicode.org> wrote:
> I believe the concurrent star of a language A is (|A|)*, where > > |A| = {x ∊ A : {x}* is a regular language} > > (The definition works for the trace of fully decomposed Unicode > character strings under canonical equivalence.) I misremembered. The notation is (/A/)*, where starter-free x ∊ A is dealt with by converting x into its maximal substrings all of whose characters are of the same canonical combining class and putting them in /A/ in place of x. > Concurrent star is not a perfect generalisation. If ab = ba, then > X = {aa, ab, b} has the annoying property that X* is a regular trace > language, but |X|* is a proper subset of X*. For Unicode, X would be > a rather unusual regular language. So this is the other way round. We will get /X/ = {aa, a, b}, so X* is a proper subset of /X/*. Richard.