Erlang runtime tools not found

 Appmon:start(). failed in erlang 21.0

I am learning Erlang OTP in these days, while trying to launch Appmon in Erlang shell I got a failure:

1> appmon:start().
* 1: syntax error before: descript
1> 

How could it be? This is supposed to be one of the Erlang system bundled runtime tools!

Okay, the tools got a revolution in Erlang 17, as described

in release note:

--- appmon --------------------------------------------------------------

   OTP-10915  Removed gs based applications and gs based backends. The
          observer application replaces the removed applications.

...
...

--- pman ----------------------------------------------------------------

   OTP-10915  Removed gs based applications and gs based backends. The
          observer application replaces the removed applications.


--- toolbar -------------------------------------------------------------

   OTP-10915  Removed gs based applications and gs based backends. The
          observer application replaces the removed applications.


--- tv ------------------------------------------------------------------

   OTP-10915  Removed gs based applications and gs based backends. The
          observer application replaces the removed applications.