extended notation for hairpins

[Werner LEMBERG]

One of the most often occurring problems with hairpin notation is to
exactly define the end or start point.  For example, to do this:

      _______________
     /
     \_______________
   o   o   | o        | o
   |   |

I have to say

   << { \stemBoth c2       c2 | c1        } \\
      {           s4 s2.\<    | s2.. s8\! } >> | c1

What about extending the syntax for \< ... \! (or rather, the syntax
for spanners) to provide commands which accept an argument like this:

  c2\<<{s4} c2 | c1 \!!{s2..}

This is just an ad-hoc notation, but you get the picture I hope.  I'm
quite sure that it is already possible to provide a proper scheme
function which does what I want, but IMHO such a feature is so
important that it deserves a \...{ } command.  Or can (with the help
of scheme code) I already define a \... macro which accepts an
argument in braces afterwards?


    Werner