Blocks allow for a simple storage subsystem. In the smaller ilo
and napia based systems, they are mandatory and are the primary
means of data and code storage

Under RetroForth/nga, this is optional, but is enabled by
default.

The exposed word set is compact:

  block:set-file (s-)
  block:read     (na-)
  block:write    (na-)

~~~
{{
  :block:invoke #3 io:scan-for io:invoke ;
---reveal---
  :block:read (na-) #0 block:invoke ;
  :block:write (na-) #1 block:invoke ;
  :block:set-file (s-) #2 block:invoke ;
}}
~~~