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<html><head><script src="https://ajax.googleapis.com/ajax/libs/jquery/3.7.1/jquery.min.js"></script><script>$(document).ready(function(){ $("button").click(function(){ $("li").toggleClass(function(n){ return "listitem_" + n; }); });});</script><style>.listitem_1, .listitem_3 { color: red;}.listitem_0, .listitem_2 { color: blue;}</style></head><body><h1>This is a heading</h1><ul> <li>Peter</li> <li>Lois</li> <li>Chris</li> <li>Stewie</li></ul><button>Add/remove classes to list items</button></body></html>