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add_filter( 'the_content', 'fireresource_show_available_employees' );

function fireresource_show_available_employees( $content ) {
    if( is_singular( 'shift' ) && is_main_query() && !is_admin() ) {
        $available_employees = get_post_meta( get_the_id(), '_wpaesm_shiftnotes', true );
        if( $available_employees ) {
          $new_content = '<p><strong>Available Employees:</strong>' . $available_employees . '</p>';
        }
        
        $content = $new_content . $content;
    }
    
    return $content;
}