const validPseudo = new Set( [ 'not', 'hover', 'active', 'checked', 'focus', 'focus-visible', 'visited', 'focus-within', 'selection' ] );

// Traverse through all the Selectors.

csstree.walk(ast, {
    visit: 'Selector', enter(node, item, list) {

        // Flag to check if the selector has a Pseudo class.
        // Set to false at the start of traversing the Selector.

        let foundValidPseudo = false;
        let pseudoClass = null;

        // Traverse through all the nodes inside the Selector.

        csstree.walk(node, {
            enter(innerNode, item, list) {

                // Check if the node is a Pseudo class.

                if (innerNode.type === 'PseudoClassSelector' || innerNode.type === 'PseudoElementSelector') {

                    // Check if the Pseudo class is valid.

                    if(validPseudo.has(innerNode.name)) {

                        // Switch flag when you hit a Pseudo class.
                        // This flag will be true for the rest of the traversal.

                        pseudoClass = innerNode.name;
                        foundValidPseudo = true;
                    }
                }

                //  Remove all nodes when valid Pseudo class is found.

                if (foundValidPseudo) {
                    list.remove(item);
                }
            }
        });
        if (foundValidPseudo) {
            log(csstree.generate(node), pseudoClass);
        }
    }
});