SOS is described in SOS.LES[S,DOC], and briefly below:

	To create a file (a program, a writeup, or whatever),
first be sure you're at monitor level (type CALL on a display
or <control>C on a teletype), then
type "CREATE file.ext" where "file.ext" is the file name and extension
that you want the file to be called (be sure to end this command
with a carriage return).   After SOS types a line
number (starting at 00100) you may
start typing whatever is to go in that file.  
When you're finished and want to save what you have, type
carriage return to finish the line you're on, and then type
alt-mode (ALT, ESC or PREFIX on a TTY, or <control><shift>K).
SOS will type an asterisk after you type alt-mode.  SOS will
now accept any of the following commands (all commands are
terminated by a carriage return):

	P<line number>		Type the specified line
	P			Type the next 15 lines
	I<line number>		Insert text starting at
				the specified number.
				To leave Insert mode, type
				alt mode.
	I*			Insert at the end of the
				current page.
	D<line number>		Delete specified line
	W			Write the file.  Saves it
				in case the system crashes
	E			Exit - save the file and exit.

To return to editing the file after you exited, type "EDIT file.ext"
SOS will type an asterisk and the commands above are valid.