abstract class information

[Wulfklaue via Digitalmars-d]

I am just looking up abstract class information and i notice that 
there is no information on dlang.org/spec

Yet using this as a resource:

https://www.tutorialspoint.com/d_programming/d_programming_abstract_classes.htm

It can be found, with a proper example. Am i blind and not seeing 
it in the specs?

Even Ali's book barely mentions it ( 55 ).

And yet abstract classes exit and function in D. So strange.