Making it easier to do the simplest kind of round-trip with XML: convert an XML string to X-expressions, manipulate, and then convert these X-expressions back to an XML string.
Take a string containg XML and break it into two X-expressions: one representing the prolog of the document, and the other representing everything under the root node. Your @racket[_xml-string] must have a root node, but it doesn't need a prolog.
Take two X-expressions representing the prolog and root of an XML document and join them back into an XML string. In other words, the inverse of the function above.