code: private void initializePossibleAttributeNames() {
          final Set<String> possibleAttribNames = new LinkedHashSet<>();
  
          for (final Map<String, List<Object>> attributeMapForSomeUser : this.backingMap.values()) {
              final Set<String> keySet = attributeMapForSomeUser.keySet();
              possibleAttribNames.addAll(keySet);
          }
  
          this.possibleUserAttributeNames = Collections.unmodifiableSet(possibleAttribNames);
      }
docs: Compute the set of attribute names that map to a value for at least one
  user in our backing map and store it as the instance variable
  possibleUserAttributeNames.