The error is reported as a warning and the predicate fails. Quietly fails if a syntax error has occurred. This behaviour is a SWI-Prolog extension. Describes the detailed layout of the term. The formats for the various types of terms if given below.
All positions are character positions.
If the input is related to a normal stream, these positions are relative to the start of the input, when reading from the terminal, they are relative to the start of the term. Arg describes the argument. Elms describes the positions of the elements. Used for a compound term not matching one of the above. FFrom and FTo describe the position of the functor.