Plan: a very basic SXML-to-XML transformer. Only takes a small, simple subset of the full SXML.

gistfile1.sls

(deffn sxml->xml (sxml)
  (with (tag (car sxml) attrs nil children nil)
    (if (= (caadr sxml) '@) ; if we have attributes
      (do (if (is-a? (car (cdadr sxml)) 'list)
            (set attrs (cdadr sxml)) ; association list attribute mode
            (set attrs (pairs (cdadr sxml)))) ; pair list attribute mode -- plan only
          (set children (cddr sxml)))
      
      (set children (cdr sxml))) ; else, if we don't have any attributes
    
    (string ~< tag
      (map attrs (fn (attr)
        (string ~\s (car attr) ~= ~" (xml-encode (cadr attrs)) ~")))
      (if children ; if we have children
        (string ~>
          (map children (fn (child)
            (if (is-a? child 'list) ; if we're a sub-tag
              (sxml->xml child) 
              (xml-encode child)))) "</" tag ~>) ; else, if we're just a text node
        " />")))) ; else, we're an empty tag; use a self-closing end tag

(deffn xml-encode xs
  (global-sub (string xs)
    ~& "&amp;"
    ~< "&lt;"
    ~> "&gt;"
    ~" "&quot;"))

GitHub's Scheme highlighter is slightly wrong for Plan; whenever you see ~", it's an instance of Plan's char type as the literal character ", but Scheme (and thus GitHub) treats it at the start of a string. Please ignore the discrepancy.