Collapse OS file system

This is a Forth-style filesystems which is very simple. It is a
list of 1024 bytes block, organised in 16 lines of 64 columns
each. You refer to blocks by numbers. You show them with LIST.
You interpret them with LOAD. For a convenient way to browse
blocks, see Block editor at B100.

Conventions: When you see "(cont.)" at the bottom right of a
block, it means that the next block continues the same kind of
contents. This of course only work for informational text.

Block numbers are abbreviated with prefix "B". "BX" means
"block X".

The master index of this filesystem is at B1.