Action trace is significantly enhanced by adding code to add_filter() in wp-includes/plugin.php
function add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1) {
...
if ( function_exists( "bw_call_lazy_trace_actions" ) ) bw_call_lazy_trace_actions( $tag, $wp_filter[$tag] ); return true; }