*procedure
**
UNRDCH
      To push a character back into the  currently  selected
input   stream  so  that  the  next  rdch  will  yield  that
character.
      res := unrdch ()
      The last character read is pushed back into  the  cur-
rently  selected  input steam ( ie., the steam is backspaced
by one character ). The routine returns a true result unless
an  attempt is made to move past the buffer origin, in which
case the result is false and unrdch has no effect  (  unless
the stream is exhausted, in which case it is true anyway ).
      The implementation of unrdch and rdch is such that  it
is always possible to perform at least one unrdch.


