
unget(+Stream)

   Back up one character on Stream

Arguments
   Stream              Integer (stream number) or Atom (reserved or user-defined                symbolic stream name).

Type
   Character I/O

Description
    Go back one character on the given Stream. This can be used to
    implement lookaheads.
    
    The number of characters that can be reliably ungotten is 4, and the
    result is only defined if these characters have been read previously.
    
    The result of the operation is undefined if
    
    trying to unget more than 4 characters
    trying to unget more characters than had been read previously
    trying to unget after a seek operation
    
    In these cases, unget/1 will succeed, but subsequent read operations
    will return undefined results.


Modes and Determinism
   unget(+) is det

Exceptions
     4 --- Stream is not instantiated.
     5 --- Stream is neither an integer nor an atom.
   192 --- Stream not in read mode.

Examples
   
% look ahead one character in Stream:

    peek(Stream, X) :-
    	get(Stream, X),
	unget(Stream).


See Also
   get / 2, get_char / 2
