diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 3bc45a7..08fc1bc 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -138,16 +138,18 @@ class Registry implements DestructableInterface {
   protected $themeManager;
 
   /**
-   * User functions grouped by the word before the first underscore.
+   * User function names grouped by the word before the first underscore.
    *
-   * @var array
+   * @var string[]
    */
   protected $groupedFunctions = [];
 
   /**
    * All user-defined functions that have been added to groupedFunctions.
    *
-   * @var array
+   * The array keys are the function names, for faster dereferencing.
+   *
+   * @var bool[]
    */
   protected $seenFunctions = [];
 
@@ -455,8 +457,8 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
         $result[$hook]['type'] = $type;
         $result[$hook]['theme path'] = $path;
 
-        // If theme hook has base hook mark its preprocess functions always
-        // incomplete to inherit base hooks preprocess functions.
+        // If a theme hook has a base hook, mark its preprocess functions always
+        // incomplete in order to inherit the base hook's preprocess functions.
         if (!empty($result[$hook]['base hook'])) {
           $result[$hook]['incomplete preprocess functions'] = TRUE;
         }
@@ -635,7 +637,7 @@ protected function postProcessExtension(array &$cache, ActiveTheme $theme) {
 
     // Gather prefixes. This will be used to limit the found functions to the
     // expected naming conventions.
-    $prefixes = array_keys((array) $this->moduleHandler->getModuleList());
+    $prefixes = array_keys($this->moduleHandler->getModuleList());
     foreach (array_reverse($theme->getBaseThemes()) as $base) {
       $prefixes[] = $base->getName();
     }
@@ -668,8 +670,8 @@ protected function postProcessExtension(array &$cache, ActiveTheme $theme) {
       }
     }
 
-    // Add missing variable preprocessors. This is needed for hooks that do not
-    // explicitly register the hook. For example, when a theme contains a
+    // Add missing variable preprocessors. This is needed for modules that do
+    // not explicitly register the hook. For example, when a theme contains a
     // variable preprocess function but it does not implement a template, it
     // will go missing. This will add the expected function. It also allows
     // modules or themes to have a variable process function based on a pattern
@@ -689,8 +691,8 @@ protected function postProcessExtension(array &$cache, ActiveTheme $theme) {
         }
       }
     }
-    // Inherit all base hook variable preprocess functions into suggestion hooks.
-    // This ensures that derivative hooks have a complete set of variable
+    // Inherit all base hook variable preprocess functions into suggestion
+    // hooks. This ensures that derivative hooks have a complete set of variable
     // preprocess functions.
     foreach ($cache as $hook => $info) {
       // The 'base hook' is only applied to derivative hooks already registered