static S htag(S tag) {
  ret htag(tag, "");
}

static S htag(S tag, O contents, O... params) {
  S openingTag = hopeningTag(tag, params);
  S s = str(contents);
  if (empty(s))
    ret dropLast(openingTag) + "/>");
  ret openingTag + s + "</" + tag + ">";
}