(get-sxml-tag argument) -> symbol or #f
If the given argument is s-xml, this will return the "tag" symbol. For any other value, returns false.
Examples:
>> xml :: (body (p "alpha" (a (@ (href . "a")) "aleph")) (p "beta")) >> (get-sxml-tag xml) :: body >> (get-sxml-tag "foo") :: #f