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    Date: 18 November 1982 09:49-EST
    From: Robert Elton Maas <REM>

    Sometimes when I ctrl-S to stop output it never comes back on even
    though I've typed new stuff which echos just fine -- what do I have to
    type to undo a ctrl-S? (I conjecture the read-eval-print loop turns it
    back on but ordinary READLINE in a user program doesn't.)
That's right, READLINE will not re-enable output. The mechanism is:

  Output is disabled by setting the variable ^W (uparrow W, not C-W) to T 
  and re-enabled by setting it to NIL.

  The control character c-W sets ^W to T.
  The control character c-V sets ^W to NIL.
  The control character c-S, like c-W, sets ^W to T, but has the additional
   effect that when seen by the reader at READ (not READLINE, TYI, etc) time
   sets ^W back to NIL. The reason that it only works at READ time is that 
   it works via STATUS MACRO; ie, (STATUS MACRO #^S) => +INTERNAL-^S-MACRO
   which is described roughly by (LAMBDA (NIL NIL) (SETQ ^W NIL) NIL).
   Note that this macro is a splicing macro and consequently causes a token 
   break. (AB) is read as (A B) while (AB) is read as (AB).
   Note further that (READLINE) does not ignore any of the characters
   , , or . This may imply that you want to write your own READLINE
   that does (a) discard  and  when it sees them and (b) set ^W back to
   NIL when it sees  and then discard the .

Hope this answers some of your questions along these lines.