Defines
symbol as a name to be used in Xuriella's sexp
representation for XSLT.
It used when XSLT in sexp syntax includes a list of the form:
(symbol ...arguments...)
The list
arguments is then destructured using the specified lambda list, and
body is invoked during compilation time as an implicit
progn.
body should return a function of one argument, which will be called
at run time with a context object as an argument.