read and buffered from the layer below back to that layer, so that it
can be re-provided to what ever is now above.
-Returns 0 on success and failure.
+Returns 0 on success and failure. If C<Popped()> returns I<true> then
+I<perlio.c> assumes that either the layer has popped itself, or the
+layer is super special and needs to be retained for other reasons.
+In most cases it should return I<false>.
=item Open
normally call the C<Open()> method of next layer down (if any) and
then push itself on top if that succeeds.
+If C<PerlIO_push> was performed and open has failed, it must
+C<PerlIO_pop> itself, since if it's not, the layer won't be removed
+and may cause bad problems.
+
Returns C<NULL> on failure.
=item Binmode
=head2 Implementing PerlIO Layers
+If you find the implementation document unclear or not sufficient,
+look at the existing perlio layer implementations, which include:
+
+=over
+
+=item * C implementations
+
+PerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core.
+
+PerlIO::gzip and APR::PerlIO (mod_perl 2.0) on CPAN.
+
+=item * Perl implementations
+
+PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN.
+
+=back
+
If you are creating a PerlIO layer, you may want to be lazy, in other
words, implement only the methods that interest you. The other methods
you can either replace with the "blank" methods