That is, C<=head1> is no longer a header; C[C<>] is no
longer code. The only exception is that C[Z<>] remains a no-op, divider entity.

Now, all entities of the format C<letter> C[<] C[>] are considered arbitrary
entities, and it is up to the user to define what they mean. Hence it is
possible to redefine them all to mean what they do in POD and you get POD back.

Entities do not have to accept C[<>] as their delimiters. It is now possible to
set a list of delimiters that will be accepted after the letter; any nested
entities will then have to use the same delimiters to be parsed as entities. You
can still use multiple of the same delimiter to surround your text: however you
do it, the nested entities must use exactly the same ones or not be parsed.

