define-extension-compiler (symbol (&rest lambda-list) &body body)
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.