wrap: func [ n /end ] [ rejoin [ "<" either end ["/"][""] n ">" ] ]

block-to-xml: func [data] [
  accumulate [ n x ] a copy "" data [
    rejoin [ a  wrap n  either block? x [ block-to-xml x ] [ x ]  wrap/end n ] ] ]

; usage

>> block-to-xml [ 'inv [ title "10-0001" date "10-10-2010" client [ name "R ltd" city "NY" ] ] ]
== {<inv><title>10-0001</title><date>10-10-2010</date><client><name>R ltd</name><city>NY</city></client></inv>}