simple-xml: convert xml to hash, use xml element directly.

Convert xml to hash, use hash-ref to get xml element directly, or use loop to traverse all the list elements.

Chen Xiao <chenxiao770117@gmail.com>

Racket build-in xml decoding function is: xml->xexpr. But decoded result is a hierrachical list, not covenient use.

Simple-xml package convert this list to hash map, you can get element(elements) directly.