multi-syntax support (Primer, Syntax, NF&R, ...)

Again, the system I set up for handling multiple syntaxes has an example
here, which I believe ought to be copied and configures as per each
document, because it provides more clarity and functionality to users
doing the switching:

   http://www.w3.org/2009/02/multisyntax-support/example-1.html

(It might need some more functionality for Syntax, which doesn't have
the the same parallel structure.  Boris?)

     -- Sandro