Limit Easy Digital Downloads Cart to One Item

Limit the cart/checkout to one item in Easy Digital Downloads.

<?php
// Limit checkout to one item only
function wd_edd_one_item_checkout( $download_id, $options ) {
    if( edd_get_cart_quantity() >= 1 ) {
        edd_empty_cart();
    }
}
add_action( 'edd_pre_add_to_cart', 'wd_edd_one_item_checkout', 10, 2 );