After noticing an error in my math book, I rewritten its section “Funcoids and filters” to reflect that .

Previously I proved an example demonstrating that , but this example is believed by me to be wrong. The example was removed from the book.

Thus I removed all references to (as it is the same as ) and reworked the chapter “Funcoids and filters” to reflect the change.

The book is available free of change at this Web page.

The story of the past:

was defined by the formula .

From the theorem in “The diagram” section (the theorem with a diagram) it trivially follows that . It follows trivially, but I have found this only today.

I’ve added to the post a note with my old definition.