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A specialisation of
that allows the user to look ahead
into the stream of items. This is useful, for instance, when reading flat
file data that contains record separator lines which are actually part of the
The detailed contract for
has to be defined by the
implementation because there is no general way to define it in a concurrent
environment. The definition of "the next read()" operation is tenuous if
multiple clients are reading concurrently, and the ability to peek implies
that some state is likely to be stored, so implementations of
may well be restricted to single threaded use.