>     >   Using characters other than * to indicate variant forms. I can
 >     >   imagine a command with two independent variants, accesed by
 >     >   \foo, \foo*, \foo+, and \foo*+
 >    this could be provided easily enough, eg, S{<symbol-to-scan} and
 >    might indeed be generally useful, any comments?
 > A very nice idea!  I like it.  But the \foo*+ form is too much -- a
 > single character should be sufficient.

you misunderstood, the suggestion was to allow scanning for some
arbitrary symbol (well sort of arbitrary) e.g.

\DeclareDocumentCommand \foo { S{*} S{+} }
   { ... }

the above would  parse

\foo  (returning \BooleanFalse\BooleanFalse)
\foo* (returning \BooleanTrue\BooleanFalse)
\foo+ (returning \BooleanFalse\BooleanTrue)

but of course also

\foo*+ (returning \BooleanTrue\BooleanTrue)

whether this is helpful / sensible to have this extension is another
matter but this would be the functionality