sS mapMethodLike hopeningTag(S tag, Map params) {
  ret hopeningTag(tag, mapToParams(params));
}

static S hopeningTag(S tag, O... params) {
  new StringBuilder buf;
  buf.append("<" + tag);
  params = unrollParams(params);
  for (int i = 0; i < l(params); i += 2) {
    S name = (S) get(params, i);
    O val = get(params, i+1);
    if (nempty(name) && val != null) {
      if (eqOneOf(val, html_valueLessParam(), true))
        buf.append(" " + name);
      else {
        S s = str(val);
        if (!empty(s))
          buf.append(" " + name + "=" + htmlQuote(s));
      }
    }
  }
  buf.append(">");
  ret str(buf);
}