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Back up one character on Stream
- Integer (stream number) or Atom (reserved or user-defined symbolic stream name).
Go back one character on the given Stream. This can be used to
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
In these cases, unget/1 will succeed, but subsequent read operations
will return undefined results.
- trying to unget more than 4 characters
- trying to unget more characters than had been read previously
- trying to unget after a seek operation
Modes and Determinism
- (4) instantiation fault
- Stream is not instantiated.
- (5) type error
- Stream is neither an integer nor an atom.
- (192) illegal stream mode
- Stream not in read mode.
% look ahead one character in Stream:
peek(Stream, X) :-
get / 2, get_char / 2