Determine current (error) location in a lazy list. True when
Location is an (error) location term that represents the
current location in the DCG list.
is a term file(Name, Line, LinePos, CharNo)
stream(Stream, Line, LinePos, CharNo) if no file is
associated to the stream RestLazyList. Finally, if the Lazy list is
fully materialized (ends in ), Location is
unified with end_of_file-CharCount.