code: @SuppressWarnings({"unchecked"})
      public <T> ExtensionList<T> getExtensionList(Class<T> extensionType) {
          ExtensionList<T> extensionList = extensionLists.get(extensionType);
          return extensionList != null ? extensionList : extensionLists.computeIfAbsent(extensionType, key -> ExtensionList.create(this, key));
      }
docs: Returns {@link ExtensionList} that retains the discovered instances for the given extension type.
  
  @param extensionType
  The base type that represents the extension point. Normally {@link ExtensionPoint} subtype
  but that's not a hard requirement.
  @return
  Can be an empty list but never null.
  @see ExtensionList#lookup