function showLogin() {
  var loginLink = document.getElementById("login_link"); 
  var loginForm = document.getElementById("login_form");
  jscss( "add", loginLink, "hidden" );
  jscss( "remove", loginForm, "hidden" );
}

function jscss(a,o,c1,c2)
{
  switch (a){
    case 'add':
      if(!jscss('check',o,c1)){o.className+=o.className?' '+c1:c1;}
    break;
    case 'remove':
      var rep=o.className.match(' '+c1)?' '+c1:c1;
      o.className=o.className.replace(rep,'');
    break;
    case 'check':
      return new RegExp('\\b'+c1+'\\b').test(o.className)
    break
  }
}