Debugging output has been added to the following files (for 5.4.10):

index.php
wp-blog-header.php
wp-config.php
wp-load.php
wp-settings.php
wp-admin/admin.php
wp-admin/index.php
wp-admin/options-general.php
wp-includes/plugin.php
wp-includes/wp-db.php

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index.php
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(dirname(__FILE__).'/wp-blog-header.php'));

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wp-blog-header.php
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(dirname(__FILE__).'/wp-load.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH.WPINC.'/template_loader.php'));

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wp-config.php
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH.'wp-settings.php'));

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wp-load.php
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH.'wp-config.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(dirname(ABSPATH).'/wp-config.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH.WPINC.'/load.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH.WPINC.'/functions.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH.WPINC.'/version.php'));

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wp-settings.php
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/version.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/load.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/default-constants.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/plugin.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/compat.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/class-wp-list-util.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/formatting.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/meta.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/functions.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/class-wp-meta-query.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/class-wp-matchesmapregex.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/class-wp.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/class-wp-error.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/pomo/mo.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('require_wp_db()'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('wp_set_wpdb_vars()'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('wp_start_object_cache()'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/default-filters.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('register_shutdown_function(\'shutdown_action_hook\')'));
if (SHORTINIT && is_callable('exelana_debug'))
  exelana_debug(exelana_describe('SHORTINIT => return false'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('load L10n library'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('load most of WordPress'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('wp_plugin_directory_constants()'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('do_action(\'muplugins_loaded\')'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_include($plugin));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('do_action(\'plugins_loaded\')'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('do_action(\'sanitize_comment_cookies\')'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('do_action(\'setup_theme\')'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('do_action(\'after_setup_theme\')'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('do_action(\'init\')'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_describe('do_action(\'wp_loaded\')'));

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wp-admin/admin.php
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(dirname( __DIR__ ) . '/wp-load.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . WPINC . '/http.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/includes/admin.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/network/menu.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/user/menu.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/menu.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/admin-header.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/admin-header.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/admin-footer.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/admin-header.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/includes/upgrade.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/admin-footer.php'));

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wp-admin/index.php
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(__DIR__ . '/admin.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/includes/dashboard.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/admin-header.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/admin-footer.php'));

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wp-admin/options-general.php
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(__DIR__ . '/admin.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/includes/translation-install.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/admin-header.php'));
if (is_callable('exelana_debug'))
  exelana_debug(exelana_require(ABSPATH . 'wp-admin/admin-footer.php')); ?>

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wp-includes/plugin.php
if (is_callable('exelana_debug'))
  exelana_debug("add_filter('$tag',".exelana_describe_fn($function_to_add).')');
if (is_callable('exelana_debug'))
  exelana_debug("remove_filter('$tag',".exelana_describe_fn($function_to_remove).') => '.var_export($r,true));

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wp-includes/wp-db.php
if (is_callable('exelana_debug'))
  exelana_debug("wpdb::__construct('$dbuser','$dbpassword','$dbname','$dbhost)'");