document.addEventListener("DOMContentLoaded", function(event) {
    main();
});

function main() {
    let links = document.getElementsByClassName('biblink');
    for (let i = 0; i < links.length; i++) {
        links[i].addEventListener("click", function(event) {
            event.preventDefault();
            let pre = this.parentNode.getElementsByTagName('pre')[0];
            pre.classList.toggle('visible');
        });
    }
}