How to limit posts to 1 from each term with tax_query?

<?php
$categories = array('economy','the-constitution','monetary-policy','liberty');
foreach($categories as $category) {
    $args = array(
        'posts_per_page' => 1,
        'category_name' => $category,
        'tax_query' => array(
            array(
                'taxonomy' => 'highlight',
                'field' => 'slug',
                'terms' => array( 'lead','featured' ),
                'operator' => 'NOT IN'
            )
        )
    );
    $wpse42358_query = new WP_Query( $args );
    while( $wpse42358_query->have_posts() ) : $wpse42358_query->the_post();
        // write post stuff here
    wp_reset_postdata();
}
?>

I opted to use WP_Query because you have more control on taxonomy queries.